/ . Embryo/, exp. Morph. Vol. 41, pp. 245-258, 1977
Printed in Great Britain © Company of Biologists Limited 1977
245
Limb-somite relationship: origin of the limb
musculature
By ALAIN CHEVALLIER, MADELEINE KIENY
AND ANNICK MAUGER 1
From the Laboratoire de Zoologie et Biologie animale,
Universite scientifique et medicate de Grenoble
(Equipe de recherche associee au CNRS n° 621,
'Morphogenese experimentale')
SUMMARY
Quail-to-chick grafting experiments performed during the third day of incubation demonstrate that somites can contribute to limb development. In orthotopic recombinations,
migrating cells originating from the grafted unsegmented or segmented somitic mesoderm
adjacent to the wing or leg field end up in the musculature respectively of the wing or the
leg, where they express exclusively myogenic properties. Thus, in these heterospecific recombinations, the anatomical muscle has a double origin: muscle bulk of somitic origin; tendons
and connective tissues of somatopleural origin. Similar features are observed in heterotopic
recombinations with (segmented or unsegmented) somitic mesoderm located cranially or
caudally to the limb levels.
In the reverse chick-to-quail grafting experiments, the somitic participation to the limb
mesoderm can also be observed. But it is less regular than that obtained in the quail-to-chick
recombinations, and the muscle bulk is made up in various proportions of graft-originated
somitic cells and of host somatopleural cells.
The possible existence of juxtaposed and interdigitated myogenic and tendinogenic compartments is discussed in view of the dissimilarity between the results of the two kinds of
heterospecific recombinations.
INTRODUCTION
The limb-bud arises as a local thickening in the somatopleural mesoderm
of the body wall at somite level 15-20 for the wing-bud and 26-32 for the legbud. These thickenings subsequently elongate along their proximo-distal axis,
i.e. perpendicularly to the cranio-caudal axis of the embryo. During this early
period of development the mesenchyme of the limb-bud looks homogeneous
and appears to be composed of a single cell type. Later, from stage 24 of Hamburger & Hamilton (1951) on, these mesenchymal cells differentiate into muscle,
cartilage, bone and connective tissue cells.
Pursuing our investigations on the relationships between the somites and the
limb-bud (Kieny, 1969, 1970, 1971, 1972), the question arose whether bird
1
Author's address: Laboratoire de Zoologie et Biologie animale, Universite scientifique
et medicale de Grenoble, 38041 Grenoble Cedex, France.
246
A. CHEVALLIER, M. KIENY AND A. MAUGER
somite cells participate in limb myogenesis in the same manner as that described in lower vertebrates [Selachians: Dohrn (1884); Braus (1899, 1906);
Borisov (1970), Teleosts: Corning (1894), Amphibians: Borisov (1970)].
While it is well established that the bird limb skeleton and the dermis (Dhouailly & Kieny, 1970, 1972; Kieny, 1960, 1971; Pinot, 1970) arise from the
somatopleural mesoderm, it is not clear yet whether the musculature is of
somatopleural or somitic origin or whether limb muscles form from a combination of somitic and somatopleural cells.
Up to a recent date, the assumption of the somitic participation in limb
musculature, hence the previous penetration of somite cells into the limb-buds,
could not be confirmed experimentally. Grafting (Hamburger, 1938) as well
as carbon-marking (Saunders, 1948) and tritium-labelling (Seichert, 1971)
experiments failed to demonstrate a somitic contribution to the development of
the limb. All these experiments were carried out at late stages, when the limbbud was, at the earliest, about to bulge out. Despite the in vitro recombination
experiments of Gumpel-Pinot (1974) which indicated that somitic material
takes part in the formation of the wing-bud, the participation of somite cells
in limb myogenesis remained unsolved.
Since the presumptive myoblasts are histologically indistinguishable from other
mesenchymal limb-bud cells at early stages of development, only longlasting
labelling experiments could reveal the origin of the limb musculature.
Using the biological cell labelling technique advocated by Le Douarin &
Barq (1969), Christ, Jacob & Jacob (1974#) on the one hand, and our group
[Chevallier, Kieny & Mauger (1976)] on the other hand, performed similar
experiments that consisted in replacing the somitic mesoderm of the wing
level of the chick host by somitic mesoderm from a quail donor. These experiments showed that indeed somitic cells participate in limb myogenesis.
The experiments reported in this paper were undertaken to answer the
following questions. (1) What is the extent of the somitic participation in the
morphogenesis of the limb musculature ? (2) Are the somite-limb musculature
relationships restricted to the somitic mesoderm of the corresponding limb
level ? (3) Are these relationships the same in both kinds of xenoplastic grafting
experiments between chick and quail ?
MATERIAL AND METHODS
The experiments were carried out on chick embryos either of a White Leghorn
strain or of a crossing of Wyandotte x Rhode Island Red and on Japanese
quail (Coturnix coturnix japonica) embryos, during the 3rd day of incubation.
The exact stage attained at theJime of operation was specified by the number
of pairs of somites.
The experimental design comprised replacing the somitic mesoderm of the
wing or the leg level of one species by somitic mesoderm from the other species
247
Origin of limb musculature
Table 1. Distribution of experimental cases according to origin and stage {in pairs
of somites) of grafted tissues and host embryos
Hosts operated on the
Grafts
Wing level
Cephalocaudal Limit stages of donors
level of
origin
Chick
Quail
Cervical
Wing
Flank
Leg
—
14-18
Limi t stages
Chick
14-23
12-20
—
10-23
20-24
12-22
13-15
18-26
15-16
16-22
Quail
Leg level
Number
of
cases
Limit stages
Chick
Quail
—
Number
of
cases
—
8
—
12-17
9
20
—
17-24
—
3
—
6
15-20
4
3
—
—
21-25
—
18-22
—
—
4
5
according to a procedure that has become routine and basic to many experiments in our group (Chevallier, 1975; Kieny, 1972; Mauger, 1912a, b). Figs.
1 and 5). When the translocations were performed at the wing level, the chick
or quail hosts ranged in developmental stages from 12 to 17 pairs of somites
(with the exception of 5 out of 50 cases ranging in stages from 20 to 23 pairs
of somites) so that unsegmented or only partially segmented somitic mesoderm
was excised; whereas, when performed at the leg level, only unsegmented somitic mesoderm was extirpated, the hosts ranging in developmental stages between 17 and 25 pairs of somites.
The grafts composed of a row of five to seven somites or presumptive somites
were obtained from the cervical region (somites 5 through 10 inclusive), the
wing region (presumptive somites or somites 15 through 20 inclusive), the
flank region (presumptive somites or somites 21-25 inclusive) or the leg region
(presumptive somites 26 through 32 inclusive). The donors ranged in developmental stages between 10 and 26 pairs of somites (Table 1).
The orientation of the graft was recorded for each operation and only aadd
homopleural operations were made on the left or right side of the embryos so
that in no case was the medio-lateral axis of the graft reversed. The operated
embryos were allowed to develop until day 6 to 12 of incubation. They were
then sacrificed, fixed in Helly's solution and immediately embedded in paraffin.
Longitudinal or transverse sections of the limbs, 7/*m thick, were stained
according to the nuclear reaction of Feulgen and Rossenbeck.
248
A. CHEVALLIER, M. KIENY AND A. MAUGER
Donor
Somite no. 14
Somite no. 14
Graft: somite
mesoderm from
level
Ectoderm
Discarded
Fig. 1. Experimental scheme of orthotopic replacement of the somitic mesoderm
from the wing level by heterospecific somitic mesoderm. Combinations between chick
host and quail donor, and vice versa.
RESULTS
In this paper, we focus our histological investigations on the intrinsic limb
musculature. The observations that were made simultaneously on the limb
girdles will be published separately (Chevallier, 1977).
A. Quail grafts to chick hosts
1. Orthotopic transplantations
Wing level. There were 20 chick embryos in which the somitic mesoderm of
the wing level was orthotopically (Fig. 1) replaced by quail somitic mesoderm.
FIGURES
2-4
Quail grafts to chick host. Fusiform muscles.
Fig. 2. Forearm muscle six days after the orthotopic replacement of the somitic
mesoderm at the wing level. Quail donor and chick host: 17 pairs of somites, x 284.
Figs. 3 and 4. Two cases of forearm muscles seven days after the heterotopic replacement of the somitic mesoderm of the wing level by somitic mesoderm from the leg
level. Quail donors: respectively 16 and 20 pairs of somites. Chick hosts: respectively 15 and 16 pairs of somites, x 672. The interrupted lines represent the limit
between chick (C) and quail (Q) cells. CT, surrounding muscular connective tissue
comprising chick cells.
250
A. CHEVALLIER, M. KIENY AND A. MAUGER
In all wings of the operated side, quail cells of somitic origin were found in the
upper arm, lower arm and hand. While the skeleton and dermis were always
and uniformly composed of chick somatopleural cells, the musculature, on the
contrary, was made of both somite-originating quail cells and somatopleural
chick cells. However, these cells were not distributed at random, but the bispecificity was highly organized.
Indeed, 4-7 days after the operation, the bulk of the fusiform muscles were
mainly composed of quail cells whereas the attached tendons, at both ends,
were constituted by chick cells (Fig. 2). Moreover, in the bulk, the presence of
quail nuclei was selectively restricted to the differentiating muscle fibres proper.
We found only one exception to this picture; namely, in one of these 17 specimens, the bulk and the tendons of one fusiform muscle from the zeugopod were
completely made up by chick cells. In the case of penniform muscles, the muscle
fibres of quail constitution laid at an angle to a central tendon, that was composed solely of chick cells (Figs. 6, 7). Finally, in the particular case of the elbow
muscles (anconeus and brachialis muscles), which are directly inserted to the
skeletal elements without intervening tendinous tissues, quail muscle fibres were
found to abut directly against the perichondrium of the humerus and of one of
the zeugopodial bones.
In the older specimens sacrificed at a total incubation time of twelve days
(three cases), the architectural arrangement of the musculature was more advanced and permitted the observations of another pattern of intermingling
between quail and chick cells. The complete muscle organ was wrapped up by
external connective tissues (perimysium) composed of chick cells. And, in the
same way, the bundles of quail muscle fibres became surrounded by the internal
endomysial connective tissues of chick constitution.
Leg level. In five cases, orthotopic transplantations were performed at the leg
level. Here also, the quail cells originating from the grafted somitic mesoderm
were exclusively found in the bulk of the muscle, whereas the tendons comprised
chick cells.
2. Heterotopic transplantations
The question arose whether the somitic mesoderm adjacent to the prospective limb region is qualitatively determined and penetrates exclusively the limb
of the corresponding cephalocaudal level or whether any somitic mesoderm
can invade a growing limb-bud, the somitic mesoderm being thus endowed
with general invading and myogenic properties.
The experimental design consisted in replacing the somitic mesoderm of the
wing level by a portion of the somitic mesoderm taken from the neck level
(eight cases, Fig. 8), flank level (six cases) and leg level (three cases, Figs. 3, 4)
(Fig. 5). These 17 translocation experiments gave the same results regardless of
the cephalocaudal origin of the somitic quail graft: cells originating from the
quail segmental plate penetrated far down to the autopodial level and in all
Origin of limb musculature
251
Donors
Grafts: somite
mesoderm from
Somite no. 14
Host
Somite no. 21.
Somite no. 24
Fig. 5. Experimental scheme of heterotopic replacement ofthesomitic mesoderm from
the wing level by heterospecific somitic mesoderm from the neck, flank and leg
levels. Combinations between chick host and quail donor, and vice versa. E, discarded ectoderm.
252
A. CHEVALLIER, M. KIENY AND A. MAUGER
&mxh
Origin of limb musculature
253
three wing segments formed exclusively the muscle fibres proper. AgainJ;he
tendinous and connective enveloping tissues comprised chick host cells.
These results are similar to those observed in the series of orthotopic transplantations and they attest that there is no somitic region alization for the limb
musculature in general.
B. Chick grafts to quail hosts
The question then arose whether the beforegoing observations correspond
to the processes of normal development. In particular, is the somitic participation due to intrinsic morphogenetic properties of the somitic cells or merely
to the developmental asynchrony between the two species involved (hatching
after 16 days for the quail and 21 days for the chick).
To answer this question, the reverse experiments were undertaken in which
chick somitic mesoderm was grafted in a quail host. Orthotopic (13 cases) and
heterotopic (7 cases) transplantations were performed at the wing and at the
leg level. As in the previous set of experiments skeleton and dermis were composed of host cells. As concerns the musculature, again the tendons and connective tissues were always composed of host cells (Fig. 9). But the muscle
fibres proper were constituted in various proportions of both chick and quail
cells (Figs. 10-12). It appeared that no muscle bulk was exclusively composed
of chick grafted somitic cells, whereas some muscles were constituted exclusively
of quail host cells. Most muscle bulks were of mixed constitution, with a majority of quail host cells. Although the graft-originated cells were scarcer in the
wings of these chick-to-quail grafting experiments than in the reverse ones,
their muscular tissue specificity could be ascertained by their exclusive presence
inside muscle bulks.
CONCLUSIONS
In birds, somites can contribute to the development of the appendages. By
means of heterospecific recombination experiments involving quail and chick
embryos we were able to show that the somitic contribution is selectively
restricted to the musculature in the developing wing or leg.
FIGURES 6-8
Quail graft to chick host. Penniform muscles.
Figs. 6 and 7. General view and detail of a forearm muscle seven days after the orthotopic replacement of the somitic mesoderm at the wing level. Quail donor and chick
host: 16 pairs of somites, x 160 and x 672.
Fig. 8. Forearm muscle eight days after the heterotopic replacement of the somitic
mesoderm of the wing level by somites from the neck level. Quail donor: 16 pairs of
somites. Chick host: 14 pairs of somites, x 672.
Fig. 9. Chick graft to quail host. Penniform muscle. Lower leg muscle ten days after
the orthotopic replacement of the somitic mesoderm at the leg level. Chick donor
and quail host: 19 pairs of somites, x 672. The central tendon (T) is composed of
chick host and the muscle fibres (Mw) are made up by quail cells.
17
EMB 41
A. CHEVALLIER, M. KIENY AND A. MAUGER
11
Figs. 10—12. Chick graft to quail host. Penniform muscles. Two neighbouring forearm muscles six days after the heterotopic replacement of the somitic mesoderm
of the wing level by somitic mesoderm of the leg level. The one (left muscle of Fig.
10 and Fig. 11) contains chick and quail (encircled areas) cells approximately in the
same proportions; the other (Fig. 12) contains mainly quail cells. Only few scattered
chick cells (arrows) can be detected. Chick donor: 19 pairs of somites. Quail host:
15 pairs of somites, x 250 and x 672.
Origin of limb musculature
255
Thus, the muscle in the anatomical sense of the term appears as a composite
structure. The tendons and other muscular connective tissues are of host, i.e.
somatopleural origin, whereas the muscle cells are of graft, i.e. somitic origin.
These results raise the question of a possible organizing role of the tendon
blastemata in the organogenesis of the anatomic muscles.
The above-mentioned features are conspicuous when the graft derives from
the faster developing quail embryo, but they are less prominent when the graft
is obtained from the slower growing chick embryo. In the case of quail somitic
grafts, the muscle bulks proper of the chick limbs on the operated side were
always exclusively composed of grafted somitic cells. Contrariwise, in the case
of chick somitic grafts, the muscle bulks of the quail wings displayed a range
of various cellular compositions. There were apparently no muscles exclusively
composed of grafted chick cells, but there were some muscles exclusively composed of host cells. Anyhow, the majority of the muscle bulks were of mixed
chick-quail constitution, with an evident predominance of quail host cells.
The invasiveness of the somitic cells certainly plays a role in this phenomenon.
The cells of the faster developing quail embryo conceivably invade the outgrowing chick limb-bud at an abnormally high rate with respect to the slower
developing chick environment, and prematurely occupy available spaces. Thus,
these quail cells come to participate in the formation of the muscles in a higher
proportion than the chick somite-originated cells do in a quail environment.
Nevertheless it must be emphasized that the participation in the muscle bulk
does not occur haphazardly, but is highly selective; the cells from the slower
growing chick also end up exclusively in the muscle fibres.
This system thus bears some resemblance to compartmentalization in insect
development (Garcia-Bellido, 1975). Indeed the quail-to-chick grafting experiments are reminiscent of those using Minute mutants in Drosophila (Morata &
Ripoll, 1975) in which the M + /M+ recombinant cells overgrow the slower
proliferating heterozygous M/M+ cells, filling up most or all of one compartment, the boundaries and extension of which thus becoming revealed. It would
appear then, that in vertebrates as well as in insects, compartments exist (cf.
Morata and Lawrence, 1977). However, the compartments of vertebrates
would be of a different nature to those of insects, not being separated from one
another by simple straight or curved lines, but rather being intimately interdigitated into one another. Consequently, the anatomical muscle would be
composed of at least two highly interdigitated compartments, one containing
prospective myoblasts, the other filled up by prospective tendinoblasts and
possibly other types of specialized fibroblasts.
The fact that the reverse combination of chick graft into quail host does not
lead to an exclusive graft origin for the muscle bulk may also reflect the postulated greater invasiveness and faster growth properties of quail cells. The latter
may originate either from the host somitic or host somatopleural mesoderm.
At present, on the basis of our experimental knowledge, both origins appear
17-2
256
A. CHEVALLIER, M. KIENY AND A. MAUGER
equally likely. On the one hand, the somitic origin may be explained by a possibly incomplete extirpation of the somitic mesoderm from the host or by a
compensatory longitudinal immigration of somitic cells from anterior or
posterior levels. On the other hand, unpublished experimental results indicate
that grafted somatopleural mesoderm may participate in limb myogenesis.
However that may be, somitic cells can participate in limb development, and
when they do so they selectively contribute to the muscle bulk.
The myotomes (and perhaps other somitic cells) not only contribute to the
limb musculature, they also give rise to the body wall musculature (Christ et al.
19746; Chevallier, 1977). For limb, as well as for trunk, myotomal derivatives,
no cephalo-caudal regionalization exists; they develop according to their new
implantation site. This does not hold for the other somitic derivatives. Indeed,
for the dermatome (Mauger, 1912 b) and for the sclerotome (Kieny et al. 1972;
Chevallier, 1977; Chevallier, Kieny & Mauger, 1976; Chevallier, Kieny,
Mauger & Sengel, 1977), the morphogenetic capacities of the somitic mesoderm
are strictly level-dependent. The translocated portions of the somitic mesoderm
develop according to their original cephalo-caudal level by forming the portion
of the spinal feather tract and that of the vertebral column with ribs and girdle
elements specific to the tested levels (Sengel, 1972).
Prior to these somite-somatopleure relationships which involve a cellular
contribution of the somitic mesoderm to the limb-bud once it is bulging out
(unpublished data), there are other relationships, that do not seem to depend
on a cellular contribution of the segmental plate to the limb-forming somatopleure. These other relationships exist during a limited period of time and are
strictly restricted to the still unsegmented somitic mesoderm adjacent to the
limb level. Their effect comprises the acquisition of qualitative limb-inducing
properties by localised portions of the somatopleural mesoderm (Kieny,
1969-72).
It is thus clearly established that, in birds, somitic mesoderm intervenes in
limb organogenesis in at least two different ways. Firstly, it exerts an inductive
influence on the adjacent limb somatopleural mesoderm, on which it confers
the capacity to initiate limb development. Secondly, it provides the outgrowing
limb-bud with a certain number of cells the progeny of which exclusively end
up as myoblasts in the musculature.
Ce memoire represente une partie de la these qui sera soutenue par A. Chevallier devant
l'Universite scientifique et medicale de Grenoble pour l'obtention du grade de docteur
d'Etat es Sciences.
It is a pleasure to acknowledge the critical reading and linguistic assistance of Professor
Philippe Sengel. We wish to thank Nicole Cambonie, Joselyne Clement-Lacroix and Josette
Ferring for expert technical assistance.
The research was supported by grants from D.G.R.S.T. and C.N.R.S.
Origin of limb musculature
257
REFERENCES
I. N. (1970). Evolution of the sources of limb development in vertebrates. /. gen.
Bioi 31, 327-336.
BRAUS, H. (1899). Beitrage zur Entwicklung der Muskulatur und des peripheren Nervensystems der Selachier. II. Die paarigen Gliedmassen. Morph. Jb. 27, 501-629.
BRAUS, H. (1906). 1st die Bildung des Skeletes von den Muskelanlagen abhangig? Morph.
Jb. 35, 240-321.
CHEVALLIER, A. (1975). Role du mesoderme somitique dans le developpement de la cage
thoracique de l'embryon d'oiseau. I. Origine du segment sternal et mecanismes de la
differenciation des cotes. J. Embryo/, exp. Morph. 33, 291-311.
CHEVALLIER, A. (1977). Origine des ceintures scapulaire et pelvienne chez l'embryon d'oiseau
(en preparation).
CHEVALLIER, A., KIENY, M. & MAUGER, A. (1976). Sur L'origine de la musculature de l'aile
chez les oiseaux. C. r. Acad. Sci. hebd. Seanc, Paris D 282, 309-311.
CHEVALLIER, A., KIENY, M., MAUGER, A. & SENGEL, P. (1977). Developmental fate of the
somitic mesoderm in the chick embryo. Symposyum on Vertebrate Limb and Somite Morphogenesis Glasgow, 1st to 4th September, 1976. Cambridge University Press. (In the Press.)
CHRIST, B., JACOB, H. J. & JACOB, M. (1974a). Uber den Ursprung der Fliigelmuskulatur.
Experimentelle Untersuchungen mit Wachtelund Huhnerembryonen. Experientia 30,
1446-1449.
BORISOV,
CHRIST, B., JACOB, H. J. & JACOB, M. (19746). Experimentelle Untersuchungen zur Ent-
wicklung der Brustwand beim Hiihnerembryo. Experientia 30, 1449-1451.
H. K. (1894). Uber die ventralen Urwirbelknospen in der Brustflosse der Teleostier.
Morph. Jb. 22, 79-98.
DHOUAILLY, D. & KIENY, M. (1970). Participation du mesoderme somatopleural du fianc
a l'edification d'un membre supplemental chez les oiseaux. C. r. hebd. Seanc Acad. Sci.,
ParisT> 271,519-522.
DHOUAILLY, D. & KIENY, M. (1972). The capacity of the flank somatic mesoderm of early
bird embryos to participate in limb development. Devi Biol. 28, 162-175.
DOHRN, A. (1884). Studien zur Urgeschichte des Wirbeltierkorpers. VI. Die paarigen und
unpaaren Flossen der Selachien. Mitt. zool. Stat. Neapel5, 161-195.
GARCIA-BELLIDO, A. (1975). Genetic control of wing disc development in Drosophila. In
Cell Patterning. Ciba Foundation Symposium 29, pp. 161-178. North-Holland: Elsevier.
GUMPEL-PINOT, M. (1974). Contribution du mesoderme somitique a la genese du membre
chez l'embryon d'oiseau. C. r. hebd. Seanc Acad. Sci., Paris D 279, 1305-1308.
HAMBURGER, V. (1938). Morphogenetic and axial selfdifferentiation of transplanted limb
primordia of 2-day chick embryos. /. exp. Zool. 11, 379-397.
HAMBURGER, V. & HAMILTON, H. L. (1951). A series of normal stages in the development of
the chick embryo. /. Morph. 88, 49-92.
KIENY, M. (1960). Role inducteur du mesoderme dans la differenciation precoce du bourgeon
de membre chez l'embryon de Poulet. /. Embryol. exp. Morph. 8, 457-467.
KIENY, M. (1969). Sur les relations entre le mesoderme somitique et le mesoderme somatopleural avant et au cours de l'induction primaire des membres de l'embryon de poulet.
C. r. hebd. Seanc Acad. Sci., Paris D 268, 3183-3186.
KIENY, M. (1970). Sur le role du mesoderme somitique dans la differenciation de la patte de
l'embryon de Poulet. C. r. hebd. Seanc Acad. Sci., Paris D 270, 2009-2011.
KIENY, M. (1971). Les phases d'activite morphogene du mesoderme somatopleural pendant
le developpement precoce du membre chez l'embryon de poulet. Annls Embryol. Morph. 4,
281-298.
KIENY, M. (1972). Interactions between somitic and somatic mesoderm during early organogenesis of the limb in the chick embryo. Collogue International sur le Developpement du
Membre. Grenoble, 28th August to 1st September.
LE DOUARIN, N. & BARQ, G. (1969). Sur l'utilisation des cellules de la caille japonaise comme
'marqueurs biologiques' en embryologie experimentale. C. r. hebd. Seanc Acad. Sci.,
Paris D 269, 1543-1546.
CORNING,
258
A. CHEVALLIER, M. KIENY AND A. MAUGER
A. (1972a). Role du mesoderme somitique dans Je developpment du plumage
dorsal chez l'embryon de poulet. I. Origine, capacites de regulation et determinisme du
mesoderme plumigene. /. Embryol. exp. Morph. 28, 313-341.
MAUGER, A. (19726). Role du mesoderme somitique dans le developpement du plumage
dorsal chez l'embryon de poulet. II. Regionalisation du mesoderme plumigene. /. Embryol.
exp. Morph. 28, 343-366.
MORATA, G. & LAWRENCE, P. A. (1977). Homoeotic genes, compartments and cell determination in Drosophila. Nature, Lond. 265, 211-216.
MORATA, G. & RIPOLL, P. (1975). Minutes: mutants of Drosophila autonomously affecting
cell division rate. Devi Biol. 42, 211-221.
PINOT, M. (1970). Le role du mesoderme somitique dans la morphogenes precoce des membres de l'embryon de poulet. /. Embryol. exp. Morph. 23, 109-151.
SAUNDERS, J. W. (1948). Do the somites contribute to the formation of the chick wing?
Anat. Rec. 100, 756.
SEICHERT, V. (1971). Autoradiographic study of the relation of somites and wing primordia
in chick embryos. Acta morph. neerl. scand. 9, 129.
SENGEL, P. (1972). Regionalisation precoce du mesoderme somitique chez l'embryon de
poulet: developpement compare du plumage et du squelette axial. Bull. Soc. zool. Fr. 97,
485-495.
MAUGER,
(Received 9 March 1977, revised 28 April 1977)