J. Embryol. exp. Morph. Vol. 22, 2, pp. 265-77, September 1969
265
Printed in Great Britain
Histogenesis of lymphoid
organs in larvae of the South African clawed
toad, Xenopus laevis (Daudin)
By MARGARET J. MANNING 1 AND JOHN D. HORTON 1
From the Department of Zoology, The University of Hull
Before embarking on a study of the lymphoid system of an amphibian larva,
it is necessary to realize that lymphoid tissue may occur in many organs of the
body and that in all immature vertebrates and in adult poikilotherms separation
of lymphoid tissue from myeloid tissue is incomplete (Yoffey, 1960). Jordan
(1938) reviewed the early work on the haemopoietic tissues of Amphibia.
Cooper (1967tf, b) and Baculi & Cooper (1967) made a thorough investigation of the lymphomyeloid and lymphoepithelial organs of the bullfrog, Rana
catesbeiana, in the branchial region of the larva and in the ventral neck region
of the adult. In the larva they found three pairs of ventral cavity bodies and
one pair of lymph glands; in the adult, propericardial bodies, procoracoid bodies,
epithelial bodies and jugular bodies were present. Cooper (1967a) reviewed the
literature on these organs and discussed their nomenclature. The ontogenetic
development of the thymus of Rana catesbeiana was described by Cooper
(19676), who also discussed its role in immunity. Recent work on larvae of the
midwife toad, Alytes obstetricans, has shown that lymphoid tissue occurs in the
thymus, spleen, dorsal coelomic lymphomyeloid organs, liver and kidney and
is also associated with the mesentery, gut, gills and lungs (Du Pasquier, 1968).
There is relatively little lymphoid tissue associated with the post-pharyngeal gut
epithelium in most amphibians; there are no Peyer's patches and no bursa of
Fabricius (Jolly, 1923; Good, Finstad, Pollara & Gabrielsen, 1966; Good, Finstad, Gewurz, Cooper & Pollara, 1967). However, Du Pasquier (1968) found
many small lymphocytic infiltrations in the intestine of Alytes obstetricans
larvae and also a large oesophageal lymphoid organ.
The present paper describes the lymphoid organs of Xenopus laevis larvae
and the sequence of their development. The work was undertaken as a preliminary investigation to our experimental studies on the ontogenetic development of certain immunological responses. The rationale is similar to that of
Hildemann & Haas (1962) who described the order of appearance of different
types of circulating blood cell in newly hatched and young Rana catesbeiana
1
Authors' address: The Department of Zoology, The University of Hull, England.
266
M. J. MANNING AND J. D. HORTON
larvae and correlated the time of appearance of small lymphocytes with the maturation of immunological competence. We are using a different animal and we are
studying fixed lymphoid tissues rather than circulating blood cells. Sterba (1950)
gives a detailed account of thymic histogenesis in X. laevis. We shall be concerned
in the present paper with correlating the development of the thymus with the
state of lymphocytic differentiation in other lymphoid organs in a morphological
study of the total lymphoid system of the maturing larva, described stage by
stage in relation to the Normal Table of Nieuwkoop & Faber (1967).
MATERIALS AND METHODS
Spawning was induced by injection of chorionic gonadotrophin into the
dorsal lymph sac of adult male and female X. laevis. The larvae were kept in
tanks of aerated tap-water at 23 ± 1° C and fed on nettle powder: the general
conditions under which they were reared followed those suggested by Henriques
(1964). Developmental stages were determined using the Normal Table of
Nieuwkoop & Faber (1967), and larvae were selected which had reached the
appropriate stage of development at the age given in the Normal Table. Larvae
from stages 42-59 were anaesthetized in MS 222 (Sandoz), fixed in Bouin's fixative, embedded in paraffin, serially sectioned at 8 /< and stained in haematoxylin
and eosin. Forty larvae were used in the investigation reported here and the
results have since been confirmed by examination of a number of animals which
were the controls from other experiments.
RESULTS
A. The lymphoepithelial and lymphomyeloid organs o/Xenopus larvae
An examination of serial sections through the head and trunk showed that
in Xenopus larvae lymphoid tissue is present in the following places:
(a) Thymus. This is a large paired organ situated between the eye and the
the ear and readily visible in the living larva. It shows typical cortical/medullary
differentiation. For details of thymic histology we shall refer to Sterba (1950).
(b) Ventral cavity bodies. There are three pairs of lymphoid accumulations in
the ventral pharyngeal region. These are similar to the ventral cavity bodies
described by Cooper (1967fl) in larvae of Rana catesbeiana. In X. laevis the
first pair are found one at each side of the ventral part of the pharynx on the
postero-lateral wall of the first branchial chamber just above part of the second
subarcualis muscle (M. subarcualis rectus ii). The second pair lie alongside the
third subarcualis muscle (M. subarcualis rectus iii) in a position close to the
opening of the second branchial chamber into the opercular chamber, at a level
where the opercular chamber opens to the exterior (Fig. 2E). The third pair
lie at the base of the cartilage of the third branchial arch in a position where they
hang between the openings of the second and third branchial chambers, again
at a level where the opercular chamber opens to the outside; for details of the
Histogenesis of lymphoid organs
267
musculature and skeleton of the pharyngeal region of X. laevis see Sedra &
Michael (1957). Sterba (1950) described, for the first time, two pairs of ventral
pharyngeal organs in X. laevis larvae; he called these 'procoracoidal bodies'
(Prokorakoidalkorper). From Sterba's description of their position, these appear
to be identical to the first and second of the pairs of ventral cavity bodies
described here, but we prefer not to use Sterba's terminology since the name
1
procoracoid bodies' refers to a different pair of lymphoid organs in certain
adult amphibians (see Cooper, 1967a). Sterba noted that the anlagen of the
ventral cavity bodies were infiltrated with' small round cells'. On morphological
criteria we believe these cells to be lymphocytes: this is supported by experimental evidence since their numbers are depleted by thymectomy (Manning,
unpublished results). We would draw attention to the location of the ventral
cavity bodies in X. laevis larvae: they are in a ' sentinel' position both in relation
to the opening of the branchial chambers into the opercular cavity and to the
opening of the opercular cavity to the outside.
(c) Spleen. The histology of the spleen of Xenopus laevis is described by
Sterba (1951). Figure 2 C, D shows the spleen of a stage-59 larva. Cells of the
white pulp include small lymphocytes, larger cells of the lymphocytic series,
a few haemocytoblasts, and cells with lobed nuclei, usually with two nucleoli, and
voluminous pale-staining cytoplasm (the ' degenerierender Makrolymphozyten'
of Sterba). This white pulp area is delineated by a boundary layer (the
' Grenzschichtmembran' of Sterba) which comprises a double layer of elongated
cells. Scattered lymphocytes extend beyond the boundary layer into the red
pulp.
(d) Kidneys. There was no lymphoid tissue in the pronephros of the Xenopus
larvae of our series. In the mesonephros there is haemopoietic tissue which
includes cells of the granulocytic and lymphocytic series. This is found along
the medial wall of the organ (Fig. 1F) and also around the nephrostomes. There
is little or no inter-tubular lymphoid tissue in the larval kidney in Xenopus.
(e) Liver. Accumulations of lymphocytes and granulocytes occur in the subcapsular region of the larval liver.
(/) Gut wall. There is little lymphoid tissue associated with the post-pharyngeal region of the larval gut. An occasional small area of diffuse lymphocytic
infiltration was seen in a few larvae but these were irregular in distribution:
there were no well-defined and consistently occurring lymphoid areas associated
with the gut epithelium, or with the lungs or any other viscera.
(g) Lymph nodes. No lymph nodes were found; the larval lymph glands described by Cooper (1967 a) in Rana catesbeiana are lacking in Xenopus laevis,
as also are the dorsal coelomic lymphomyeloid organs of the larval midwife
toad (Du Pasquier, 1968).
(h) There is no bone marrow in the larvae.
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M. J. MANNING AND J. D. HORTON
B. Histogenesis of the lymphoid organs in relation to
Nieuwkoop and Faber's stages of development
Stages 42-44. The first lymphoid organ to develop is the thymus. Figure 1A
shows the thymic anlage at stage 42, the earliest stage of our series. The thymus
comprises relatively few cells and is still in continuity with the pharyngeal epithelium of the second branchial pouch—the region from which it is derived.
Anlagen of the other lymphoid organs are not yet detectable in our material.
Stages 45 and 46. Detachment of the thymic bud from the pharyngeal epithelium occurs during stage 45. The thymic cells are large and few in number;
most are immature lymphoid cells with pale nuclei and prominent nucleoli,
some are in mitosis. Yolk inclusions are still present. At stage 45, the anlage of
the spleen can be distinguished as a well-defined condensation of cells in the
dorsal mesogastrium.
Stage 47. The thymus is detached from the pharyngeal epithelium and fibroblasts are condensing around the organ to form a distinct capsule; in some stage47 larvae the line of separation from the pharynx is indicated by the remains of
a strand of epithelial cells. Mitotic figures are frequent, but the thymic cells are
still relatively large in size; no small lymphocytes have yet appeared. The spleen
shows the beginning of differentiation into future red and white pulp as evidenced by the presence of a few erythrocytes amongst the red pulp cells; cells
of the future white pulp are large immature lymphoblasts.
Stage 48. Figure IB shows the thymus of a stage-48 larva: it is clearly encapsulated and completely separated from the pharyngeal epithelium. There
is still no clear differentiation into cortex and medulla, although in some
stage-48 larvae the future cortical cells can be distinguished from the medullary
epithelial cells by their smaller size and more deeply staining nuclei. Mitotic
figures are frequent and the average size of the thymic cells has decreased as
their numbers are increasing. There are as yet, however, no cells of small
lymphocyte dimensions. Lymphocytic differentiation is still absent from the
spleen where all the cells of the white pulp are of large size and immature appearance (Fig. 2 A). Immature haemopoietic tissue is seen along the medial wall of
the mesonephros for the first time at stage 48.
Stage 49. Figure 1C shows the thymus of a stage-49 larva. By comparing
Fig. 1C with Fig. 1 B, it can be seen that considerable growth and differentiation
of the thymus has occurred between stages 48 and 49. In the stage-49 larvae of
our series the distinction between thymic cortex and thymic medulla is obvious.
The basophilia of the cortex is due to the presence of large numbers of lymphocytes, including, for the first time, many small lymphocytes. Melanophores
have appeared in the cortex. Most of the epithelial cells of the thymic medulla
are pale-staining and have large nuclei with prominent nucleoli. The epithelial
derivatives described by Sterba (1950), such as Hassal's corpuscles and cystic
spaces, are not yet prominent; nevertheless a few medullary cells show cyto-
Histogenesis of lymphoid organs
269
plasmic eosinophilia and an occasional small cystic space is seen. The spleen
increases in size between stages 48 and 49. In the splenic red pulp the predominant cell is the reticular cell with its elongated nucleus. In the white pulp
the cells are rounded and lymphoid in appearance but they are all of large or
medium lymphocyte size; there are no small lymphocytes present. The anlagen
of the ventral cavity bodies can be distinguished for the first time at stage 49:
their presence is marked by the hypertrophy of a few cells of the pharyngeal
epithelium. Under this small patch of cells the blood vessels are enlarged and
there are lymphatic spaces. At stage 49 these spaces contain an occasional
medium-sized lymphocyte. In later larval stages, lymphocytes penetrate into
the subepithelial connective tissue and come to form a lymphocytic accumulation which extends up to the single layer of epithelial cells bordering the
pharyngeal lumen; there are no lymphocytes in the subepithelial tissue at stage
49 however. Lymphomyeloid tissue is seen for the first time in the liver at
stage 49: a few lymphocytes of small size, together with cells of the granulocytic
series, are found occasionally, in very small numbers, at the periphery of the organ.
Similar small accumulations of cells of the granulocytic and lymphocytic series are
seen in the haemopoietic tissue of the medial wall of the mesonephros at stage 49.
Stage 50. By stage 50 the cortex of the thymus contains many small lymphocytes. Pigmentation is conspicuous in the cortex, the melanophores extending
inwards as far as the cortico-medullary boundary. There are now small lymphocytes scattered amongst the epithelial cells of the medulla. Some of the medullary
epithelial cells seem to be degenerating with the formation of cystic spaces and,
in some larvae, the early formation of unicellular and multicellular Hassal's
corpuscles was detected: these derivatives of the thymic epithelial cells were
identified from the descriptions of Sterba (1950). From this time onwards the
thymic medulla is often eccentric: the cortex does not completely surround the
medulla but forms a broad cap over it dorso-laterally. The spleen of a stage-50
larva is shown in Fig. 2B. During stage 50, lymphoid differentiation of the splenic
white pulp takes place. In early stage-50 larvae the white pulp still comprises
mainly large and medium-sized lymphocytes, but in more advanced stage-50
larvae (including that shown in Fig. 2B) many small lymphocytes are present. By
late stage 50' degenerating macrolymphocytes' (the' degenerierender Makrolymphozyten' of Sterba, 1951) are seen in the splenic white pulp and in these larvae the
boundary layer (' Grenzschichtmembran' of Sterba, 1951) is apparent, the cells of
the boundary layer being attenuated cells with elongated nuclei as in the adult
spleen. In less mature larvae of stage 50, and also in a few individuals of stage 49,
immature cells of the boundary layer are distinguishable; these are plump cells
with larger nuclei than in the mature state. A ventral cavity body of a stage-50 larva
is shown in Fig. 2F. Small and medium-sized lymphocytes have appeared in the
organ; they are seen in blood vessels, in lymphatic channels, and in the subepithelial lymphocytic accumulation. By stage 50 groups of small lymphocytes are seen
at the edges of the liver; they are particularly noticeable in the anterior part of
270
M. J. MANNING AND J. D. HORTON
the organ. In the mesonephros, small lymphocytes appear around the nephrostomes for the first time at stage 50.
Stage 51. By stage 51 the thymus is fully differentiated: the cortex is broad
with many small lymphocytes; unicellular and multicellular Hassal's corpuscles
and cystic spaces are present in the medulla. The spleen has increased considerably in size between stage 50 and stage 51. The boundary layer clearly delimits
A
\>7
'- a
Histogenesis of lymphoid organs
271
the white pulp, which is now fully lymphoid with many small lymphocytes.
The ventral cavity bodies have also increased greatly in size. At stage 51 they
are conspicuous lymphoid organs with many small lymphocytes, both in the
subepithelial lymphocytic accumulations and in the underlying blood vessels
and lymphatic channels. By stage 51 therefore the lymphoid organs of the larva
have completed their histogenesis. In the liver and in the mesonephros the
proportion of lymphocytic elements in relation to the size of the organ reaches
a level at stage 51 which remains roughly similar in all subsequent larval stages.
The liver of a stage-51 larva is shown in Fig. IE.
Stages 52-59. In the thymus the medulla shows increasing numbers of epithelial derivatives (Hassal's corpuscles, cystic spaces, etc.). By stage 54 these
are very prominent (see Fig. 1D). Figure 1D also shows that the thymic medulla
is eccentrically placed and that lymphocytes accumulate outside the thymus
itself and are in close proximity to the pharyngeal epithelium in the region
where the medulla is not surrounded by cortex. These extra-thymic lymphocytes
and similar lymphocytic accumulations near the subthymic and brachial veins
were observed in a number of larvae from stage 50 onwards. In other individuals,
however, although serial sections of the thymus of both sides were studied, no
extra-thymic accumulations were seen. Sterba (1950) noticed these extra-thymic
lymphocytes and believed that they were emigrating from the thymus. In the
spleen growth proceeds with branching of the central white pulp until 3-5 white
pulp areas are seen in sections of the mature larval spleen (more are present in
the adult). Figure 2C shows the spleen of a stage-59 larva. The ventral cavity
bodies reach their maximum size around stage 56 (see Fig. 2G). During metamorphosis they disappear when the branchial apparatus is lost.
DISCUSSION
Our present study of the larval lymphoid tissues of X. laevis shows that
lymphoid differentiation in the thymus precedes that in other lymphoid organs.
A similar situation occurs in other vertebrates: this is discussed in the review of
FIGURE 1
(A) Pharynx of a stage-42 larva in the region of the left second branchial
pouch: the thymic bud is still in continuity with the pharyngeal epithelium. (B)
Thymus of a stage-48 larva: the thymus is now detached, ph.e., Pharyngeal epithelium; ///., thymus.
(C) Thymus of a stage-49 larva showing cortico-medullary differentiation.
(D) Thymus of a stage-54 larva, showing the fully differentiated cortex and medulla.
This section also shows an extra-thymic accumulation of lymphocytes in the area
of the pharyngeal epithelium. e-th.L, Extra-thymic accumulation of lymphocytes.
(E) Liver of a stage-51 larva: groups of lymphocytes can be seen at the periphery.
b.s., Liver sinuses with blood cells;/., lymphocytes \p.c, cells of the liver parenchyma.
(F) Mesonephros of a stage-59 larva, showing lymphocytes in the medial wall.
a., Wall of the dorsal aorta; /., lymphocytes; /., cells of the kidney tubule.
272
M. J. MANNING AND J. D. HORTON
Good & Papermaster (1964). Thus the thymus is the first lymphoid organ to
develop in man (Hammar, 1905, 1921), in the opossum (Block, 1967) and in the
chicken (Papermaster & Good, 1962). As early as 1894 Beard, from his study of
the skate, suggested that the thymus is the source of lymphoid cells which
Histogenesis of lymphoid organs
273
ultimately become distributed to peripheral lymphoid organs. In X. laevis we
find that small lymphocytes are abundant in the thymus by stage 49, but do not
appear in the spleen or in the ventral cavity bodies until stage 50. By stage 51
the lymphoid organs of X. laevis have completed their lymphoid histogenesis.
The thymus by this stage shows many Hassal's corpuscles: these medullary
derivatives apparently occur at an earlier stage in the thymus of X. laevis than
in that of Rana sylvatica, where, according to Fabrizio & Charipper (1941), they
first appear during metamorphosis.
X. laevis possesses relatively few larval lymphoid organs in comparison with
the bullfrog, Rana catesbeiana, and with the midwife toad, Alytes obstetricans.
In particular, it lacks any organized lymphoid organ equivalent to the larval
lymph glands of bullfrogs (Cooper, 1967 a), or to the dorsal coelomic bodies and
oesophageal organ of larval midwife toads (Du Pasquier, 1968). It would appear
that the adult Xenopus also lacks some of the lymphoid organs possessed by
other anurans (Sterba, 1950; R.J.Turner, personal communication): in particular, it lacks the lymph nodes recently described in adult Bufo marinus (Kent,
Evans & Attleberger, 1964; Evans et al. 1965) and in adult Rana catesbeiana
(Cooper, 1967#; Baculi & Cooper, 1967). This absence of lymph nodes in
Xenopus may be correlated with deficiencies of the adult in its immune response to
injections of bovine serum albumin (Horton, 1968). Thus in Bufo marinus the
lymphoid tissue of the lymph nodes, as well as that of the kidney, spleen and liver,
responds to antigenic stimulation by proliferation and by the appearance of antibody-forming cells (Diener & Nossal, 1966; Cowden, Gebhardt & Volpe, 1968).
FIGURE 2
(A) Spleen of a stage-48 larva, e., Erythrocytes in future red pulp; w.p., future
white pulp.
(B) Spleen of a stage-50 larva showing the white pulp with many small lymphocytes.
The white pulp is delimited by the boundary layer. Arrows to boundary layer cells.
(C) Spleen of a stage-59 larva showing the distribution of red pulp and white pulp.
r.p., Red pulp; w.p., white pulp.
(D) One of the white pulp areas shown in (C) seen at a higher magnification. Small
lymphocytes predominate within the white pulp and also extend beyond the
boundary layer into the red pulp. Arrows to boundary layer cells, d.m., 'Degenerating
macrolymphocyte' (Sterba, 1951); //., nucleus of a haemocytoblast.
(E) Pharynx of a stage-49 larva in the region of the right opercular opening showing
the anlage of the second right ventral cavity body, br.a., Cartilage of the third
branchial arch; o.br.c, opening of the second branchial chamber into the opercular
cavity; o.o.c, opening of the opercular chamber; s.m., third subarcualis muscle;
v.c.b., anlage of second right ventral cavity body.
(F) Second ventral cavity body of a stage-50 larva: lymphocytes are now present in
the organ, e.b.v., Erythrocyte in blood vessel; l.b.v., lymphocyte in blood vessel;
Lie, lymphocyte in lymphatic channel; l.s.e., subepithelial accumulation of lymphocytes; o.c, opercular chamber; s.m., third subarcualis muscle.
(G) First ventral cavity body of a stage-56 larva: the organ now contains many
lymphocytes and has reached its maximum development.
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M. J. MANNING AND J. D. HORTON
From our present study of the histogenesis of the lymphoid organs in X.
laevis it will be noted that lymphoid transformation in the ventral cavity bodies
differs from that in the thymus and spleen. Thus the ventral cavity bodies become
lymphoid organs at stage 50 with the appearance of small and medium-sized
lymphocytes but without the prior appearance of immature lymphoid cells of
the large lymphoblast type. In this respect, the histogenesis of the ventral cavity
bodies follows a pattern similar to that described by Ackerman (1966) for the
appendix and the tonsil iliaca of the rabbit. It is possible that small lymphocytes
migrate into organs of this type, having been formed elsewhere in the body.
Fichtelius, Finstad & Good (1968) have recently suggested that non-follicular
lympho-epithelial infiltrations associated with the gut epithelium of poikilothermic vertebrates may be micro-organs functionally equivalent to the bursa
of Fabricius of birds. Our present study shows that such lympho-epithelial
tissue is irregular in its occurrence in the post-pharyngeal gut of X. laevis
larvae, and that the only regularly occurring organs which are possible candidates for the role of'bursa equivalents' are the pharyngeal ventral cavity bodies.
However, their relatively late appearance, the way in which lymphoid transformation occurs and our unpublished observations on their thymic dependence
makes it unlikely that the ventral cavity bodies are central lymphoid organs.
We have as yet no data on the time of first appearance of immunoglobulinproducing cells in X. laevis or any evidence concerning any possible influence
of the ventral cavity bodies on the development of the antibody-producing
system. Thus the question of whether or not the ventral cavity bodies fulfil some
part of the function attributed to the bursa of Fabricius in birds remains undecided. For a discussion of the status of the bursa of Fabricius and its equivalents, see the reviews of Warner & Szenberg (1964) and of Good, Gabrielsen,
Peterson, Finstad & Cooper (1966).
The location of the ventral cavity bodies suggests to us that their function
may be to respond to foreign substances which could gain entry via the opercular opening, or which may be present in the water current passing across the
gills. Thus there is a danger to a filter-feeding animal, such as the larva of X.
laevis, from pathogenic micro-organisms which may be trapped along with the
food particles and held in the pharynx for some time before encountering the
digestive enzymes further back in the gut. A functional relationship of this sort,
correlated with the need to recognize and deal with foreign organisms invading
the pharyngeal epithelium from the trapped food mass, may also account for
the presence of the lymphoid accumulations, thought by Salkind (1915) to be
thymic organs, which occur in the pharynx of the ammocoete larva of the lamprey. Since the ancestral vertebrates were also filter-feeders and may well have
possessed pairs of primitive thymic organs which were separated from the
pharyngeal lumen by only a single layer of epithelial cells, these considerations
may be relevant to the question of the origin of the thymus.
Histogenesis of lymphoid organs
275
SUMMARY
1. In larvae of Xenopus laevis the organs in which lymphocytes are found
consistently are the thymus, spleen, ventral cavity bodies, liver and mesonephros:
larval lymph glands are lacking.
2. The ventral cavity bodies of X. laevis are described and discussed.
3. A study of the histogenesis of the lymphoid organs correlated with Nieuwkoop & Faber's stages of development shows that lymphoid differentiation in
the thymus precedes that in other lymphoid organs: by stage 49 the thymus
shows obvious differentiation into cortex and medulla, small lymphocytes
being present in the cortex.
4. Small lymphocytes do not appear in the spleen or in the ventral cavity
bodies or, in any numbers, in the liver and kidney until stage 50.
5. Lymphoid transformation is complete in all the larval lymphoid organs
by stage 51.
RESUME
Histoge'nese des organes lymphoides dans la larve du Xenopus laevis Daudin
1. Dans les larves de Xenopus laevis, les organes dans lesquels des lymphocytes ont ete trouves de facon constante sont: le thymus, la rate, les corps
cavitaires ventraux, le foie et le mesonephros. 11 n'y a pas de ganglions lymphatiques larvaires.
2. Les corps cavitaires ventraux de X. laevis sont decrits et leur signification
est discutee.
3. Une etude de l'histogenese des organes lymphoides en correlation avec
la table de developpement de Nieuwkoop et Faber, montre que la differenciation lymphoide dans le thymus precede celle des autres organes lymphoides:
au stade 49, le thymus montre une differentiation evidente en cortex et medulla,
de petits lymphocytes etant presents dans le cortex.
4. De petits lymphocytes n'apparaissent pas dans la rate ni dans les organes
cavitaires ventraux ni, du moins en nombre appreciable, dans le foie et le rein,
avant le stade 50.
5. La transformation des lymphoides est complete dans tous les organes
lymphoides larvaires au stade 51.
We should like to thank Mrs Hilary Dooks for excellent technical assistance. One of us
(J.D.H.) was supported by a Science Research Council Research Studentship.
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{Manuscript received 13 December 1968)