/. Embryo!, exp. Morph. Vol. 42, pp. 79-92, 1977
Printed in Great Britain © Company of Biologists Limited 1977
79
Application of a morphological time
scale to hereditary differences in prenatal mouse
development
By DOUGLAS WAHLSTEN 1 AND PATRICIA WAINWRIGHT 1 ,
Department of Psychology, University of Waterloo, Canada
SUMMARY
A standardized morphological time scale for prenatal mice is presented, which is useful
from 130 to 170 days chronological age with an accuracy of 01 day. Morphological age for
an embryo or fetus is shown to correlate highly with ages estimated from body weight and
crown-rump length. The time scale is used to study the comparative development from 14
to 17 days prenatal age of an F 2 and four inbred mouse strains. The F2 mice average 0-5 day
ahead of C57BL/6 mice, and C57BL/6 mice average 0-5 day ahead of A, BALB/c, and, at
some ages, DBA/2 mice. Using reciprocal Fx crosses and reciprocal backcrosses, it is also
shown that both the fetal heredity and the maternal environment contribute significantly to
the more advanced development of hybrid mice.
INTRODUCTION
The research presented in this paper addresses a problem common to all
studies of hereditary variation in the development of a single trait in mice. When
a single characteristic, for example, maturity of the brain, is compared across
several mouse strains, all of which are of the same chronological age, significant
differences are generally observed. However, it is also true that measures of many
different organ systems may all show that one strain is advanced compared to
another, and hence that the organism as a whole is at a more advanced stage of
development. Once it is known that hereditary differences in overall maturity
exist, similar differences in measures of particular organs may be quite trivial.
It is therefore important that measures of certain organs, such as the brain,
be compared in mice of different strains which are equated for their overall
maturational status. When this is done, any significant hereditary differences
observed may be of great importance for the proper understanding of neural
ontogeny. This principle has been applied fruitfully to analysis of the ontogeny
of cleft palate (Trasler, 1965; Shih, Trasler & Fraser, 1974) and forebrain fibre
tracts in mice (Wahlsten, 1975 a).
Previous studies have observed substantial hereditary variation in development
1
Authors' address: Department of Psychology, University of Waterloo, Waterloo, Ontario,
Canada N2L 3GI.
6-2
80
D. WAHLSTEN AND P. WAINWRIGHT
of the whole mouse in the pre-implantation period (McLaren & Bowman, 1973),
at the time of palate closure (Trasler, 1965; Shih et ah 1974), and in the neonatal
period (Garrard, Harrison &Weiner, 1974; Wahlsten, 19756;Wainwright, 1977).
The magnitudes of the differences detected were generally quite large considering
the rapidity of changes occurring during the various phases of ontogeny. From
the second to the fourth day of gestation, embryos from C57BL/McL females
were about 4-5 h ahead of those from C3H/BiMcL females (McLaren & Bowman,
1973). At around 15 days chronological age, C57BL fetuses were approximately
\-\ day ahead of A/Jax fetuses (Trasler, 1965). At 32 days chronological age
(about 13 days after birth) inbred strains differed by as much as 1 day, and they
lagged behind F 2 hybrids by 1-3 days (Wahlsten, 19756; Wainwright, 1977).
Because of the profound differences in the properties of the mouse at these
different stages of development, measures of developmental status have been
based upon grossly different characteristics, e.g. cell number prior to implantation, external morphology in the late pre-natal period, and behaviour in the
neonatal period (Wahlsten, 1974). The most important attribute of the properties
chosen to measure developmental age is that it should be a measure of the whole
mouse. In the fetal period of primary interest in the present study, body weight,
crown-rump length and external morphology have been found useful. Several
scales for morphological age have been presented previously (Griineberg, 1943;
Rugh, 1968; Trasler, 1965) which are useful for determining the approximate
developmental age of the embryo or fetus, but they rely on different morphological traits at different ages, and they do not allow precise quantification of
developmental age in fractions of a day. Furthermore, they have not been based
on a standardized, replicable population of mice.
The present study describes a morphological time scale for mice which is
useful from 13-0 to 17-0 days chronological age and can estimate morphological
age of an individual fetus to an accuracy of 0-1 day. It is standardized on a
population of F 2 hybrid embryos and fetuses from ¥x female mice. The population
has high fitness and can be easily replicated. Data on body weight and crownrump length are also presented for purposes of comparison. The time scale is
applied to study strain differences in developmental rate, and then the separate
contributions of fetal and maternal heredity are examined.
MATERIALS AND METHODS
Mice
All of the parent mice were obtained from the Jackson Laboratory, Bar
Harbor, Maine, at 7-8 weeks of age. The highly inbred strains A/J, BALB/cJ,
C57BL/6J and DBA/2J; and the F x hybrid B6D2FJJ (C57BL/6J) female x
DBA/2J male) were used in different experiments as described below. All animals
were housed in standard plastic mouse cages with free access to food and water
under a 12 h light - 12 h dark schedule.
Morphological timescale and hereditary differences
81
Breeding
One male and two or three females were mated per cage at 60 days of age, and
the females were checked regularly for the presence of a vaginal plug. In the
first two experiments plug checks were conducted every 4 h, and the males and
females were together only during the light phase of the day. In the third experiment (Wainwright, 1974) checks occurred every 12 h, and the mice were together
constantly. When a plug was detected, the female was isolated in a clean cage
and left undisturbed until sacrificed. The beginning of gestation (0 day, 0 h) was
defined as the midpoint of the interval between the time when a plug was detected
and the previous plug check.
Measurement of embryo and fetus
At the appointed hour (± \ h) the female was sacrificed by either cervical
dislocation or a CO2 overdose. The embryos or fetuses were carefully removed
from the uterus, freed of all membranes and then immersed in fixative (Bouin's
in the first two experiments, Carnoy's in the third). After they had been transferred to 50 % ethanol for at least a week, body weight was measured to 0-001 g.
In the first two experiments each animal was photographed at 1 x magnification,
and crown-rump length and morphological features were assessed from a
5 x 7 in. print of the animal. In the third study crown-rump length and morphological features were determined by inspection using a dissecting microscope.
RESULTS
(A) The morphological time scale
The time scale was standardized on F 2 offspring from primiparous B6D2F]_
females mated with B6D2F! males. A series of litters was obtained, one at each
day from 110 to 18-0 days chronological (gestation) age, in which the litter
sizes ranged from 9 to 13. Eight animals from each litter were randomly chosen
for examination.
(i) Morphological changes with age
After careful inspection, an animal from each litter was selected which was at
the median stage of development for that litter. That series of eight embryos
and fetuses became the basis for the morphological time scale (see Fig. 1). In
conjunction with published descriptions of morphological changes (Gruneberg,
1943; Rugh, 1968; Trasler, 1965), standard features of the eye, ear, skin and
limbs were determined for each age. These criteria, given in Table 1, proved to
be very similar to those specified by Gruneberg (1943) and Rugh (1968), with the
exception of the eye, which in the albino mice studied by Rugh seemed to be
retarded by about \ day compared to eyes in the present study. These results
also confirmed the work of Theiler (1972), who employed very similar methods
12
13
14
15
16
GESTATION AGE(days)
17
18
Fig. 1. Series of mice that serve as the visual standard for the morphological time scale described verbally in Table 1. Gestation
age, or chronological age, is timed from the midpoint of the interval between detection of a vaginal plug and the previous
plug check, and it is accurate within about 2 h. The fetuses are from an F 2 cross of C57BL/6J and DBA/2J inbred strains of mice.
11
r1cm
B6D2F2/J
H
o
H
W
in
r
oo
Morphological timescale and hereditary differences
83
Table 1. Criteria for judging morphological age on the basis of skin,
limb, eye and ear characteristics
Age
11
Skin features
Facial arches
Limb features*
Limb buds F and
Eye features
Ear features
Small white dot
Not evident
Clear recess. Dorsal
edge pigmented
Round. Cornea
faint. Ring of
pigment
Round. Distinct
cornea. Dark
pigment
Almond-shaped.
Beginning to close
Distinct recess. No
pinna protrusion
Distinct meatus and
pinna protrusion
Distinct pinna flap.
Almost over meatus
Closed. Eyelids
light
Closed. Distinct
pinna flap
Closed. Eyelids
dark. Very fine
wrinkles.
Closed. Well-fused.
Dark skin
TT
12
Whisker follicles
13
Large head follicles.
Three or two
evident
Follicles on the
ventral abdomen
14
15
Follicles on head.
Abdomen wrinkled
16
Neck and limbs
wrinkled
17-18 Skin dark. Temporal area of head
wrinkled
rl
Footplates
paddleshaped, smooth
F footplate indented. H smooth
F digits separated.
H digits webbed
Digits splayed.
Clear phalanges
on F. Digits separated on H
Digits parallel.
Clear phalanges
FandH
Like 16, but limbs
larger
Pinna growing out
and forward
* F, forelimb; H, indicates hindlimb.
to devise a developmental age scale for the prenatal mouse. The agreement
between Theiler's criteria and our own is excellent, with our standards (Fig. 1)
being about one-quarter day advanced relative to his. This is reasonable because
we used hybrid mothers, whereas his were inbred mothers with hybrid offspring.
Theiler's study is the best available description of prenatal mouse development,
although our time scale is still more convenient to use as well as being standardized
and of known reliability.
To employ the time scale for a mouse embryo or fetus of unknown age, each
feature (eye, ear, skin, limbs) is given a score in days, and then the average of
the four scores constitutes the morphological age. When a morphological feature
has a status in between the standards given for exact days, fractional scores are
assigned to the nearest 0-25 day based upon degree of resemblance of the feature
to the standard. Because each feature is measured to 0-25 day, the mean of the
four measures is accurate to roughly 0-1 day (0-25/V4). Subsequent practice has
shown this time scale to be convenient and accurate from 13-0 to 160 days
morphological age. Distinctions between 16 and 17 days are not very reliable,
and no morphological features can presently distinguish 17- from 18-day-old
fetuses.
Interobserver reliability of the scale was determined for the two authors on
the basis of scores assigned to 71 embryos and fetuses ranging from 13 to 17 days
84
D. WAHLSTEN AND P. WAINWRIGHT
B6D2F2/J
v-5-27-0-86.v+ 004 x1
20
10
1—30-27-5-I8.Y + 0-27.Y2
12
13
14
15
16
Gestation age (days)
17
18
Fig. 2. Body weight (g) and crown-rump length (mm) for each mouse in a litter at
eight successive ages timed from the onset of gestation (chronological age). Mice
are from an F 2 cross of C57BL/6J and DBA/2J inbred strains, and they are the
population from which the standard embryos in Fig. .1 were drawn. Quadratic
regression equations of best fit are given for the interval from 12 to 17 days chronological age.
morphological age which were subjects from a subsequent experiment. Each
observer independently assigned a score to each animal on the basis of a 5 x 7 in.
photographic print of the fixed embryo or fetus, and neither observer knew the
age or strain of any animal. The Pearson correlation between the scores given
by the two observers was 0-96, which demonstrates that the scale is highly reliable
when used by experienced observers.
(ii) Body weight and crown-rump length
Body weight and crown-rump length are shown for each embryo and fetus in
Fig. 2. Except for those 17 or 18 days old, great uniformity in both scores was
observed. Quadratic regression equations fitted to the data (see Fig. 2) showed
that a smooth curve gives an excellent fit. However, the utility of the quadratic
curve does not imply that development proceeds according to a second order
function (see Epstein, 1974; Goedbloed, 1972); rather, it means that a restricted
period of growth can be described accurately by a quadratic equation relating
age to size.
This standard series of litters was used to derive regression equations for
Morphological timescale and hereditary differences
85
predicting the developmental age of an embryo or fetus from a measure of its
weight or crown-rump length. The mean body weight of each litter was regressed
upon its chronological age, giving the curve of best fit, Y = 12-14 + 9-89^
— 5-09Ar2, where Jf is body weight in grams and Fis age in days. The goodness of
fit of this curve was 0-959, when the data from ages of 12-17 days were employed.
Growth outside of this range was not described well by the quadratic equation.
Fortunately, the period of interest in subsequent experiments was 14-17 days
after conception. For crown-rump length (X), the regression equation derived
was Y = 7-80 +0-729^-0-0135X 2 with a goodness of fit of 0-988.
These equations express the smooth relationship between size and age for a
standard population of B6D2F2 embryos and fetuses. When the weight of any
fetus of unknown age or strain is plugged into the appropriate equation, the
result is the age at which the average B6D2F2 fetus in the standard population
achieved the same degree of physical growth, and hence it is referred to as ' bodyweight age' measured in B6D2F2-equivalent days. Using this method, the
relative ages of two unrelated groups of mice can be compared precisely with
reference to a common standard.
(B) Strain differences in development
Two litters of each of the inbred strains A, BALB/c, C57BL/6 and DBA/2, and
the F2 hybrid B6D2F2, were obtained at each of four chronological ages; 14-0,
15-0, 16-0 and 17-0 days. All embryos and fetuses were assessed for body weight,
crown-rump length and morphological age. Morphological ratings were done
from 5 x 7 in. photographic prints by the first author using a double-blind coding
procedure. Because litter sizes were unequal, unweighted means analysis of
variance (Winer, 1962) was used to assess statistical significance. In most
instances, the difference between the two litters in each group was not significant,
and the two litters were simply pooled to form one group for analysis. The
median group size was 15, and all groups except BALB at 14 days had nine or
more subjects.
(i) Morphological age
The mean morphological age for each group is shown in Fig. 3. Differences
between strains were highly significant (F = 156-0, D.F. = 4/242, P < 0-001),
and there was a significant interaction between strain and chronological age
(F = 5-0, D.F. = 4/242, P < 0-0001). The scores for the B6D2F2 mice replicated
closely the values expected from the standard population, thereby indicating
both the reliability of the scale itself and the great predictability of morphological
development for B6D2F2 mice. C57BL/6 mice generally lagged behind the F 2
group by 0-5 day, although the difference at 14 days was not significant. A and
BALB/c mice were approximately 1 day behind the F 2 group at all chronological
ages. DBA mice were significantly ahead of A and BALB at 14 and 16 days, and
they were always behind C57BL/6.
86
D. WAHLSTEN AND P. WAINWRIGHT
17
17
16
16
£ 15
o
14
14
16
15
17
17
14
Chronological age
Fig. 3. Mean morphological age and body-weight age for four inbred mouse strains
and the F 2 hybrid cross of C57BL/6J and DBA/2J. Each point represents the mean
of offspring from two litters at each age timed from the onset of gestation. Morphological age was derived from photographs of each embryo using the standard series
in Fig. 1 and descriptions in Table 1. Body-weight age was derived using the quadratic
regression equation given in Fig. 2.
14
15
16
The extent of variability within a group can be seen in Fig. 4, which gives
morphological age for each fetus at 16-0 days chronological age. Clearly, most
fetuses of any one group fell within a range of 0-5 day.
Taken altogether, these results demonstrate that it is important to determine
precisely the morphological age for each embryo or fetus. Hereditary differences
are quite large, and variability within a litter as well as between litters of the
same strain and age is sometimes substantial.
(ii) Body weight age and crown-rump age
Ages based on body weight and crown-rump length were derived for each
fetus using the equations and procedure described previously (A ii). The pattern
of results was very similar to that for morphological age; for each measure,
differences between strains and their interactions with chronological age were
highly significant (P < 0-001).
Correlations between the three kinds of age measures on the same fetus were
high. Because changes in scores with age were so large, correlations were calculated over a restricted range of morphological ages in order that results would
not reflect merely the gross changes with chronological age. All 67 fetuses of
the five groups at 16 days chronological age served as the bases for calculations. Pearson correlations were as follows: morphological age with bodyweight age, r = 0-91; morphological age with crown-rump age, r = 0-89;
body-weight age with crown-rump age, r = 0-96.
Thus, developmental ages based upon either external morphology, body
weight or crown-rump length are highly reliable and reveal similar patterns of
Morphological timescale and hereditary differences
87
B6D2F,
»
• •• • • ••
A
..Hi.
BALB/c
C57BL/6
l t
\
t
DBA/2
i
14
15
Morphological age
16
Fig. 4. Morphological age for individual mice in each of thefivegroups at 160 days
chronological age.
differences between five groups of mice. In one respect, morphological age is
more easily interpretable. Because all kinds of mice, regardless of their adult
form or behaviour, pass through identical morphological stages, they differ only
as to when the eyes close or the follicles appear. Body weight and crown-rump
length, on the other hand, vary widely in adults of different mouse strains. This
may lead to ambiguity; it may not be clear whether a mouse is small at a particular
age because it is developmentally retarded or because it does not normally
achieve a large size (see Fuller & Geils, 1972).
The difference between morphological age and crown-rump length age for
each embryo or fetus averaged only 0-06 day for the B6D2F2 mice over all
chronological ages, further demonstrating the excellent validity of the quadratic
equation using crown-rump length (section A ii). Only the DBA group among
the inbreds had an average discrepancy of more than 0-1 day.
Developmental age estimated from body weight averaged 0-22 day behind
morphological age for the F 2 mice, and similar discrepancy was apparent in the
inbred groups. This suggests that the constant a0 = 12-14 in the regression
equation using body weight could with justification be adjusted to a0 = 12-36
for future use.
On the basis of the present findings, the equations suggested for use in other
experiments are:
F(days) = 12-36 + 9-89JT- 5-09X2,
where Z i s body weight (g); and
F(days) = 7-80 + 0-729X-0-0135X2,
where ^ i s crown-rump length (mm).
88
?
D. WAHLSTEN AND P. WAINWRIGHT
tf
DBA x DBA
C57xC57
=1
iMl at
~|
16 days
chronological age
DBAxC57
1
C57 x DBA
DBAxF,
1
C57 x F,
F^DBA
F,xC57
1
F.xF,
14 5
i
i
150
15 5
i
160
Morphological age
Fig. 5. Mean morphological age at 160 days chronological age for offspring of the
inbred strains C57BL/6J and DBA/2J, their reciprocal F x hybrids, reciprocal backcrosses, and the F 2 cross. The strain of the female parent is given on the left and male
parent on the right.
(C) Contributions of maternal and fetal heredity
The large difference in morphological development between B6D2F2 mice and
their inbred relatives C57BL/6 and DBA/2 could have arisen because of a
retarding effect of inbreeding mediated by either the maternal environment or
the fetal heredity. The respective contribution of these two influences to heterosis
were examined using reciprocal F x crosses and reciprocal backcrosses (Wainwright, 1974). Two litters of each of the crosses listed in Fig. 5 were procured
at 16*0 days chronological age, and their morphological ages were assessed by
careful examination using a dissecting microscope and criteria for morphological
development given by Rugh (1968).
(i) Morphological age
The mean morphological ages of the nine groups at 16-0 days chronological
age are shown in Fig. 5. Contributions of various factors were assessed using
planned, orthogonal comparisons, a summary of which is given in Table 2.
Comparison of inbred and hybrid fetuses in inbred mothers revealed that
hybrids were 0-45 days on the average ahead of inbreds. Based on reciprocal
backcrosses, it was found that backcross fetuses in a hybrid mother were 0-57
days ahead of backcross fetuses in an inbred mother. Finally, fetuses with a
C57BL/6 mother were in general 0-47 days ahead of those from DBA/2 mother.
These comparisons clearly revealed significant effects of both fetal and maternal
heredity on morphological age, the sizes of the two effects being quite similar.
Morphological timescale and hereditary differences
89
Table 2. Magnitude of difference in morphological age for
three major sources of variation*
Effect
Inbred versus hybrid
Fetal heredity
Inbred versus hybrid
Maternal heredity
Maternal strain
Comparisons!
DBA x DBA
DBA x C57
+
versus
+
C57 x C57
C57 x DBA
DBA x F x
Fx x DBA
+
versus
+
C57 xFx
Fx x C57
DBA x DBA
C57 x DBA
+
+
DBA x C57 versus C57 x C57
+
+
DBA x Fx
C57 x Fx
Difference
0-45 day
0-57 day
0-47 day
* All fetuses were at 160 days chronological or gestation age.
t For each cross, the strain of the female parent is given first and the strain of male parent
second. C57 indicates C57BL/6J and DBA indicates DBA/2J.
It is also important to note the similarity between scores shown in Fig. 5 and
those at 16 days of age shown in Fig. 3. Morphological ages of the F 2 groups
were virtually identical, and the differences between the inbreds C57BL/6 and
DBA/2 were nearly the same as well. The inbreds' fetuses in the present experiment averaged one full day behind F 2 , whereas those (C57 and DBA) in the
previous experiment averaged two-thirds day behind F 2 . Thus, the results of the
two experiments were closely comparable in spite of differences in experimenter
and method of scoring.
(ii) Body weight and crown—rump length
Body weight and crown-rump length were also analysed using orthogonal
comparisons. Both variables revealed significant effects of inbred versus hybrid
fetal heredity, inbred versus hybrid maternal heredity and C57 versus DBA
maternal strain. The difference in body weight between inbred and hybrid fetal
heredity (0-085 g) was somewhat greater than the analogous difference between
maternal heredities (0-057 g).
DISCUSSION
On the basis of the present findings, several conclusions can be drawn.
(1) The developmental age of a fixed mouse embryo or fetus can be measured
accurately and reliably in the late prenatal period using a simple, four-item
morphological age scale. Ages based on skin, limbs, eye and ear maturity can
often be estimated to an accuracy of 0-25 day, and the average of the ages from
the four morphological criteria is therefore accurate to nearly 0-1 day. Developmental or morphological age derived in this manner represents the chronological
90
D. WAHLSTEN AND P. WAINWRIGHT
age at which the standard population, an F 2 cross of C57BL/6J and DBA/2J,
attains the same degree of morphological maturity.
(2) Developmental age can also be measured accurately on the basis of body
weight and crown-rump length using quadratic regression equations. Body
weight and crown-rump length of fixed embryos and fetuses are highly correlated with each other as well as with morphological criteria.
(3) The rate of morphological development of the mouse in the prenatal
period is strongly influenced, and to an approximately equal degree, by both
fetal and maternal heredity, with inbreeding retarding development.
(4) Although hybrid offspring from hybrid mothers appear to be consistently
advanced compared to inbreds at all ages, relative maturity of inbred strains is
not constant, and a strain which is behind another in the prenatal period may
surpass it in the postnatal period (see Wahlsten, 19756). Overall development
does not proceed according to a smooth curve, but rather it goes in spurts and
lags which appear to be influenced by the organism's heredity.
These conclusions have important implications for both methodology and
theory.
Assessment of developmental or morphological age of the whole fetus is very
necessary, even when interest focuses on a more restricted range of characteristics.
In a recent study of the schedule of brain fibre tract ontogeny in several inbred
mouse strains, great variability appeared both within and between strains when
the schedule was described with respect to chronological age, but a very precise
schedule which was nearly identical for all strains resulted when morphological
age was used instead (Wahlsten, 1975 a). The correlation between brain maturity
and morphological age at 16 days chronological age was +0-85. A finding of
hereditary variation in development of some part of the nervous system cannot
be correctly interpreted unless data on the whole mouse are also available. This
problem is evident in the experiment by Vaughn et al. (1975), who observed both
spinal reflexes and synapses to be precocial in C57BL/6J fetuses compared to
certain other inbred strains. Lacking a measure of overall maturity of the whole
mouse, the authors could not fully determine the meaning of the strain differences.
Interpretation of research, in which some teratogen or other intervention is
applied to several strains at the same chronological age, is made nearly impossible
without knowledge of overall maturity of the embryos or fetuses. Many teratogenic effects occur only during a precisely defined critical period. Thus, if one
strain is found to be more sensitive to a teratogen than another, it may be that
the insensitive strain is precocial and has simply grown out of the critical period
for susceptibility. It is thus imperative that the teratogen be applied to embryos
or fetuses from various strains which are all at the same morphological age. The
time scale described in the present study should make it easy to do this in the
period of 13-17 days prenatal, but further work using the same research design
will be necessary in order to elucidate the course of ontogeny at earlier and
later periods.
Morphological timescale and hereditary differences
91
The time scale should also prove useful for assessing with great precision the
retardation of development by many teratogens as well as nutritional deprivation
or stress. Because the development of grossly different features such as body
weight, eyes and follicles is expressed in a common scale of measurement,
B6D2F2-equivalent days, repeated-measures analysis of variance can be employed
to assess whether some treatment has differential effects on specific organ systems
or exercises a retarding effect on overall growth. It will enable the experimenter
to determine precisely whether specific organ systems not utilized for the time
scale are retarded or advanced in certain strains compared to the progress of
overall development. Phenomena of this nature may be of great importance for
peculiarities observed in certain kinds of adult mice (e.g. Trasler, 1965). Hereditary variation in the relative timing of events in prenatal ontogeny may
underlie strain differencies in adult morphology or even behavior. The differences
may be quite small, and hence a precise measure of developmental status must
be used.
In addition to its important methodological implications, hereditary variation
in the time course of mouse development is interesting in itself. The present
study analyses the phenomenon by separating embryonic and maternal contributions to hybrid vigor using a population of two inbred mouse strains. By studying
larger numbers of strains and litters within a strain, it should be feasible to
evaluate theories of heterosis which assert that inbreeding disrupts developmental
homeostasis or buffering (Lerner, 1970; Hyde, 1973; Kabay & Trasler, 1972).
If this theory is correct, then inbreeding should cause greater differences among
litters within an inbred strain, greater variation within a litter, and perhaps
greater asymmetry in ontogeny of the two eyes, ears or limbs of the same fetus.
Similar research on the development of the nervous system may lead to discovery
of the bases for hereditary variation in adult behavior.
The authors are grateful to Kathy Bernhardt, David Comber and Brenda Young for their
technical work on these experiments. This research was supported in part by grant A0398
from the National Research Council of Canada.
REFERENCES
EPSTEIN, H. T. (1974). Phrenoblysis: special brain and mind growth periods. I. Human brain
and skull development. DeviPsychobiol. 7, 207-216.
FULLER, J. L. & GEILS, H. D. (1972). Brain growth in mice selected for high and low brain
weight. Devi Psychobiol. 5, 307-318.
GARRARD, G., HARRISON, G. A. & WEINER, J. S. (1974). Genetic and environmental factors
determining the morphological maturation of the mouse and its relationship with weight
growth. /. Embryol. exp. Morph. 31, 247-261.
GOEDBLOED, J. F. (1972). The embryonic and postnatal growth of rat and mouse. I. The
embryonic and early postnatal growth of the whole embryo. A model with exponential
growth and sudden changes in growth rate. Ada anat. 82, 305-336.
GRUNEBERG, H. (1943). The development of some external features in mouse embryos.
/. Hered. 34, 88-92.
92
D. WAHLSTEN AND P. W A I N W R I G H T
J. S. (1973). Genetic homeostasis and behavior: analysis, data and theory. Behav.
Genet. 3, 233-246.
KABAY, M. E. & TRASLER, D. G. (1972). Embryonic variability in mice: the effects of maternal
genotype on variability of morphological rating at the time of palate closure. Teratology
5, 259.
LERNER, 1. M. (1970). Genetic Homeostasis. New York: Dover.
MCLAREN, A. & BOWMAN, P. (1973). Genetic effects on the timing of early development in
the mouse. /. Embryol. exp. Morph. 30, 491-498.
RUGH, R. (1968). The Mouse. Its Reproduction and Development. Minneapolis: Burgess.
SHIH, L.-Y., TRASLER, D. G. & FRASER F. C. (1974). Relation of mandible growth to palate
closure in mice. Teratology 9, 191-202.
THEILER, K. (1972). The House Mouse. Development and Normal Stages from Fertilization to
4 Weeks of Age. Berlin: Springer-Verlag.
TRASLER. D. G. (1965). Strain differences in susceptibility to teratogenesis: survey of spontaneously occurring malformations in mice. In Teratology Principles and Techniques (ed.
J. G. Wilson & J. Warkany), pp. 38-55. Chicago: University of Chicago Press.
HYDE,
VAUGHN, J. E., HENRICKSON, C. K., CHERNOW, C. R., GRIESHABER, J. A. & WIMER, C. C.
(1975). Genetically-associated variations in the development of reflex movements and
synaptic junctions within an early reflex pathway of mouse spinal cord. /. comp. Neurol.
161, 541-554.
WAHLSTEN, D. (1974). A developmental time scale for postnatal changes in brain and behavior
of B6D2F2 mice. Brain Res. 72, 251-264.
WAHLSTEN, D. (1975a). Prenatal development of the brain in several mouse strains. Behav.
Genet. 5, 109-110.
WAHLSTEN, D. (19756). Genetic variation in the development of mouse brain and behavior:
evidence from the middle postnatal period. Devi Psychobiol. 8, 371-380.
WAIN WRIGHT, P. E. (1974). Heterosis and the rate of prenatal development in the laboratory
mouse. Unpublished M.A. thesis, University of Waterloo, Ontario, Canada.
WAINWRIGHT, P. E. (1977). Differences in the maternal performance of inbred and hybrid
laboratory mice (Mus musculus). Unpublished Ph.D. thesis, University of Waterloo,
Ontario, Canada.
WINER, B. J. (1962). Statistical Principles in Experimental Design. New York: McGraw-Hill.
(Received 16 August 1976, revised 20 June 1977)