Book - An Introduction to the Study of Embryology 3

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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.
Haddon 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

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Chapter III. Formation of the Mesoblast

As has been previously mentioned, a middle or third germinal layer early makes its appearance in ova between the epiblast and the hypoblast, which is known as the mesoblast or mesoderm.

Although the mesoblast is probably phylogenetically younger (that is, arose later in the evolution of the primitive Metazoa) than the gastrula stage, it not unfrequently, so to speak, is developed precociously ; and throughout the animal kingdom the mesoblast may often be recognised very early in development. This is why it has been unavoidable to entirely omit any reference to the mesoblast when dealing with segmentation and gastrulation. In reading this chapter, it must be remembered that the formation of the mesoblast is synchronous with the phenomena previously dealt with.

There has been considerable difficulty in comprehending the nature of the mesoblast, owing to the fact of its diverse origin in the embryos of various animals ; but, thanks to numerous recent researches, it is now possible to arrive at a more definite conclusion.

It is necessary to bear in mind that two entirely distinct structures are included under the single name of mesoblast or mesoderm; these have been termed - mesenchyme- [Hertwig] and - mesothelium - [Minot]. For the sake of clearness these will be considered apart.

1. Origin of the Mesenchyme

In the embryos of a number of forms, amoeboid cells are budded off during the blastula stage, either from the epiblast or the hypoblast, or from both layers. Minot has proposed the term “ mesamoeboids - for such wandering cells, instead of the more cumbersome titles of “mesenchyme germs - or “ primitive mesenchyme cells - of Hertwig.

Mesenchyme alone is present in Sponges; the mesoderm consisting in this group of mesamoeboids derived in the adult from the endoderm cells, although it is stated to arise from the ectoderm in the embryo.

In the Coelenterata the mesoderm may be represented only by the structureless lamella, as in Hydra ; or by gelatinous tissue in which are scattered stellate cells (mesamoeboids) mostly of hypoblastic origin in the Scyphomedusae, and mainly of epiblastic origin in the Ctenophora.


Fig. 48. - Blastula of Echinus; drawn from the living embryo. [After Metschnikoff.]

A. Mesamoeboids arising from the hypoblastic pole of the blastula. B. Later stage ; the blastula is ciliated. bl. blastoccel ; m. mesamoeboids.

During, or even anterior to, the invagination of the archenteron in Echinoderms (fig. 16), mesamoeboids are budded off from the incipient hypoblast (fig. 48). These cells wander throughout the segmentation-cavity and adhere to all the organs as they are formed, thus forming a mesoblastic investment.

In the Platyhelminth Lineus obscurus, Hubrecht has recently shown that the mesamoeboids arise during the gastrula stage from the epiblast and hypoblast (fig. 49), but mainly from the latter ; and it is probable that the truly mesoblastic organs are derived solely from the latter (see p. 165). In Leptoplana, four primitive mesoblast cells are segmented from the four yolk-hypoblast cells, and very soon they come to be situated at the lips of the blastopore. As the epiblast grows over the yolk-hypoblast (fig. 50), the meso


Fio. 49. - Origin of Mesenchyme in Lineus. [After Hubrecht.]

A. Gastrula stage ; mesamoeboids are seen arising from the hypoblast. B. Later stage, in which the epiblast is giving origin to mesamoeboids. a. archenteron ; ep. epiblast ; hy. hypoblast ; m. mesoderm.


blasts, which now appear as four bands, pass to the upper pole and obliterate the segmentation-cavity. The large amount of yolk present in the hypoblast has clearly exerted a disturbing influence upon the origin of the mesoblast.

In the Discophora the mesoblast early forms two bands, which arise from cells which must be regarded as yolk-hypoblast.


Fig. 50. - Gastrulatton and Origin of Mesoblast in Leptoplana Tremellaris. [After Hallez. ]

hi. blastopore ; ep. epiblast, ciliated in C ; hy. yolk cells (primitive hypoblast) ; m. mesoblast.


Stellate mesoblast cells, which may be considered as mesencliymatous, traverse the space between the epiblast and archenteron in the free-swimming larvse of some Polychsete Worms (Serpula) before the true coelom is developed. Similar cells are also to be found in the pre-oral lobe of embryo Oligochsetes.


In the Mollusca, as a whole, the mesoblast is derived from cells intermediate in position between the epiblast and the hypoblast (fig. 1 8), but which may be considered as belonging to the latter rather than to the former.

The presence of mesenchyme in any of the higher Metazoa must for the present be regarded as an open question.

2. Origin of the Mesothelium. - Paired outgrowths from the archenteron, which ultimately become constricted off as closed sacs, make their appearance on the completion of the gastrula stage in such diverse groups of the Metazoa as the following : - All the Echinodermata ; the Chaetognatha (Sagitta) ; Bracliiopoda (fig. 51); Peripatus (fig. 69); Balanoglossus ; and Amphioxus


Fig. 51. - Four Stages in the Development of Argiope. [After Koiccilevsky.]

A. early gastrula stage; B, C. illustrating the development of the archenteric diverticula ; C. stage after the larva has become divided into three segments.

bl. blastopore ; cce. archenteric diverticula ; me. mescnteron ; f. provisional setae.


(fig. 56). The cavity of these sacs will form the body-cavity or coelom of the adult, and the walls constitute such mesothelial tissues as the peritoneum, mesentery, muscles, and the excretory and generative organs.

Amongst the Echinodermata a pair of such diverticula usually arise from the blind end of the archenteron; sometimes only a single vesicle is constricted off, which immediately divides into two. The former is probably the more primitive mode. These two sacs enlarge and lie one on each side of the archenteron (fig. 5 2) ; the left further gives rise to the third vesicle, which by radial prolongations develops into the ambulacral system of these animals. The two remaining sacs eventually increase in size, so as to fill up the whole of the segmentation-cavity. The alimentary canal thus comes to be surrounded by the two vesicles : when these meet each other in the middle line, their applied walls become more or less absorbed, the remains forming the mesentery of the adult, and the conjoint cavities constitute the coelom or body-cavity proper. It will be remembered that there exists a layer of mesenchyme between the epithelium of the body-cavity (mesothelium) on the one hand, and the epiblast of the body-wall and the hypoblast of the alimentary canal on the other.

With the exception of not giving rise to an ambulacral system, and a possible absence of mesenchyme in some, the formation of the coelom is practically identical in the above-mentioned forms with that of the Echinoderms.

Conn has recently stated that in Serpula, which appears to have


Fig. 52. - Three Larval Stages of the Star-Fish (Asterias).

A. Late gastrula stage, with commencing archenteric diverticula. B. Coelomic pouches constricted off. C. Early larval stage, with stomodaeum not yet opening into the mesenteron : the left coelom has formed the rudiment of the ambulacral system.

a. anus (persistent blastopore, bp .) ; amb. primitive ambulacral vesicle ; a.v. anal ring of cilia ; arc. archenteron (mesenteron in C.) ; b.c. right, and b'.c'. left coelomic sac ; int. intestine ; m. mouth (stomodaeum) ; ms. mesamoeboids ; p.o. pre-oral ciliated band.

a more simple development than most other Chsetopods, the mesoblast arises at the posterior end of the elongated blastopore. At first stellate mesenchyme cells are formed which stretch across the segmentation-cavity, and some of which enclose a small posterior vesicle (anal vesicle). The remaining mesoblast cells rapidly give rise to two bands of cells, one on either side of the alimentary canal, and extending forwards to the mouth: these ‘‘mesodermal bands- segment and become hollow, thus forming the many-chambered body-cavity, and giving rise to the usual mesoblastic structures. In one Earthworm (Lumbricus trapezoides) the mesoblast is partly derived from “ mesoblasts - which are distinguishable before the segmentation-spheres are arranged into distinct layers ; but Kleinenberg inclines to the view that they are epiblastic in origin. The mesoblasts by cell-division form a pair of latero-ventral mesoblastic bands, which further develop as in Serpula. As the development of the Oligochoeta is undoubtedly abbreviated, the origin of the mesoblast is consequently liable to be modified.

In the Fresh- water Oligochsete Bhvnchelmis (Euaxes), as a considerable amount of yolk is present, the gastrulation is epibolic. The chief portion of the mesoderm arises very early from two mesoblasts, which are derived from the primitive hypoblast cells. The two mesoblastic bands occur at the junction of the epiblast with the hypoblast (fig. 53).

There is considerable uniformity in the accounts of the origin of the mesoblast amongst the Crustacea. It may be formed by paired


Fig. 53. - Gastrulation and Formation of Mesoblast in Khynchelmis (Euaxes). [After Kowalevs/ci/.]

A. Section through blastula stage of twenty cells. B. Late blastula stage, with commencing mesoblast. C. Epibolic gastrula with paired mesoblastic bands. ep. epiblast ; hy. yolk, or primitive hypoblast ; to . mesoblast.


proliferations from the hypoblast cells of the neck of the archenteron during gastrulation (fig. 54), or from one or a pair of cells which, in the blastula stage, occupy a position between the future epiblast and hypoblast, and which sink into the segmentation-cavity. It is probable that the latter case is merely a precocious abbreviation of the former. The presence of mesenchyme in this group is not yet satisfactorily established, though Beichenbach has described the development of what he terms “ secondary mesoblast - from the hypoblast cells of the Crayfish (fig. 54, f) on the completion of the gastrula stage.

The origin of the mesoblast in the Tracheate Arthropoda is still obscure. In Insects it is partly derived from a ventral groove of the epiblast, and in Spiders from an analogous solid keel. The latter is probably a modification of the former process, and Balfour has homologised the mesoblastic groove of Insects with the blastopore of a vanished gastrnla stage. In both groups the mesoblast appears to be added to by cells arising from the yolk-hypoblast. A pair of mesoblastic bands soon appear, much as in the Chaetopods, which similarly segment, each segment containing a portion of the coelom.


A. Section through part of oosperm during segmentation. B and C. Longitudinal sections during the gastrula stage. I). Highly magnified viuw of the anterior lip of blastopore, to show the origin of the primary mesoblast from the wall of the archenteron. B. Two hypoblast-cells to show the intra-cellular digestion of yolk-spheres. F. Hypoblast-cells giving rise endogenously to the secondary mesoblast.

a. archenteron ; b. blastopore ; c. central yolk mass ; ec. epiblast ; en. hypoblast ; 7i. nuclei ; p. pseudopodial process; p.ms. primary mesoblast ; s. ms. secondary mesoblast ; w.y. white yolk ; y. yolk spheres ; y.p. yolk pyramids.


Fig. 55. - Diagrammatic Representation of an ideal Gastrula Stage of an Insect at the Time when the Archenteric Diverticula are Formed. [After 0. and R. Hertwig.]

bl. blastopore ; ep. epiblast ; hy. mesenteric hypoblast ; m. parietal or somatic layer of mesoblast - between this and the hypoblast is the visceral or splanchnic layer of mesoblast ; 71 . nerve cord ; y. yolk-cells or primitive hypoblast.


In all the invertebrate groups the mesoblast mainly arises from cells which grow inwardly from the lip of the blastopore. In closely allied forms the primitive cells vary from an apparently epiblastic to an apparently hypoblastie, or to an intermediate place of origin. The extreme variations may be neglected, as being in all probability of only secondary significance.

Since this was in type Sedgwick has shown that the somites in Peripatus (fig. 69) do not directly arise as archenteric diverticula, but are separated from a pair of mesoblastic bands as in Chsetopoda. The somites are at first ventro-lateral in position, but soon acquire a dorsal extension and divide into two parts. The dorsal parts come into contact above the enteron, but do not unite with their fellows ; anteriorly they are early obliterated, but persist posteriorly as the generative glands. The ventral moieties remain distinct, and consist of a small vesicle situated in the base of the appendages, leading from which is a small coiled tube (nephridium), which acquires an external opening. The Hertwigs have interpreted the formation of the mesoblast in Insects in terms of archenteric diverticula (fig. 55), but the undoubtedly primitive character of Peripatus renders its development especially important. Although the cavities of mesoblastic bands and archenteric diverticula are homologous, their exact relation to one another is somewhat obscure.

Whatever views may be held as to the precise position of the Chaetognatha, Bracliiopoda, and Balanoglossus, the presence of archenteric diverticula in these


Fig. 56. - Transverse Sections of Embryos of Amphioxus. [After Hatsche/c.]

A. Section through the first somite or primitive segment of an embryo in which the fifth somite is being formed. B. Section through the same region of an embryo with eight somites. C. Section through the centre of the body of an embryo with eleven somites.

al. mesenteron ; be. coelom ; m. muscle fibres ; n. neural plate and canal ; nch. notochord.


forms proves that it occurred in several of the primitive Worms ; so it maybe safely concluded that the mesoblast (for the most part, at all events) of the Gepliyrea, Polyzoa, and Nematoda belongs to this category.

It will probably be shown that mesothelial mesoblast occurs also in all Mollusca. It is probable that the pericardium of this group represents the true body-cavity of other orders ; but even if this is the case, there would be a marked preponderance of mesenchyme over mesothelium in the mesoblast.

There are not sufficient data to come to a definite conclusion concerning the exact nature of the mesoblast of the Platy helminths.


Origin of the Mesoblast in the Chordata

There appears to be no valid reason for refusing to accept Bateson -s conclusion that Balanoglossus is a persistent representative of an early stage in the evolution of the Chordata from the unsegmented Worms. He has extended the observation of others that the mesoblast in this remarkable form is derived from archenteric diverticula in a manner very similar to that which is characteristic of the Echinodermata. But the details of mesoderm formation in this form and in the Tunicata must be passed by.

In Amphioxus the formation of the mesoblast is of remarkable simplicity. The development of this form (p. 29) was traced to


Fig. 57. - Three Larval Stages of Amphioxus.

[From Claus, after Hatschek.]

D. Stage with two somites (primitive segments), seen in optical longitudinal section.

E. Stage with nine somites, seen from above, showing the asymmetry of the segments. F. Living larva with mouth and first gill-slit, seen from the left side; the second, fourth, and sixth bent lines represent respectively the posterior boundary of the first, second, and third somite of the opposite (right) side.

Bl. ventral blood-vessel ; Ch. notochord ;

D. intestine ; K. gill-slit ; MF. unsegmented mesoderm fold ; N. neural canal ;

0. mouth ; Oe. anterior orifice of neural canal ; Us. somites.


an elongated gastrula stage with a dorso-posterior blastopore. Two small pouches soon arise from the archenteron (fig. 56, a) near the anterior end of the embryo, one on each side of the median dorsal line. These are followed by others, which are successively developed from before backwards (fig. 57, D, e). These extend laterally along the dorsal side of the embryo ; but, as seen in fig. 57, E and F, they are not placed symmetrically opposite one another.

The archenteric diverticula very shortly become constricted off from the archenteron, or mesenteron, as it should now be termed (fig. 56, b), and form a series of closed sacs (mesoblastic somites or primitive segments). Each somite encloses a distinct cavity or coelom. The somites gradually extend in a ventral direction, enclosing the alimentary canal (mesenteron) (fig. 56, c) ; and by the subsequent fusion of their cavities form the small coelom or body-cavity of the adult.

The outer layer of the somites is known as the somatic or peripheral mesoblast, the inner layer being termed the splanchnic or visceral mesoblast. The dorsal moities of the somites lose their cavities, and become transformed into the great lateral muscle of the larva and adult ; but the primitive segmentation is permanently retained.

It is readily apparent (fig. 56, a) that the mesoblast is derived from two regions of the hypoblast. The ventral layer is continuous with the digestive portion of the hypoblast ; while the dorsal half is derived from the axial hypoblast. The remainder of this latter is converted into the notochord (fig. 56, B, nch). The separation of the somites and the notochord from the archenteron appears to be due to the dorsal growth and coalescence of the digestive hypoblast below these structures. The cavity of the archenteron equals that of the mesenteron + the coeloms of the mesoblastic somites.

There would seem to be no mesenchymatous elements in the mesoblast of Amphioxus, unless the pair of “hinder-pole mesoderm cells- (fig. 23) are to be regarded as such. They arise from the hypoblast at the ventral lip of the completed gastrula, and are stated by Hatschek to give rise solely to the caudal mesoderm.

The origin of the mesoblast in the Newt (Triton) is very instructive, as it serves to elucidate the formation of the mesoblast in Eeptilia and to reduce the latter to the type of Amphioxus. On the completion of the gastrula stage the mesoblast is only to be found close to the blastopore (fig. 58). The main portion grows out as a pair of lateral sheets dorsal to and at each of the blastopore (fig. 58, B, m). The brothers Hertwig at first described the mesoblast as composed from the commencement of two distinct layers, the outer growing from the epiblast of the lips of the blastopore, and the inner from the primitive hypoblast. Each lateral sheet is, however, at first a solid mass of cells, which gradually extends forwards and downwards, i.e., anteriorly and ventrally. According to Scott and Osborn, the lateral mesoblast also increases at the expense of the yolk-hypoblast. The mesoblastic sheets very early split into two layers, an external somatic and an internal splanchnic. The cavity between the two layers extends ventralwards, and forms the body-cavity or coelom. The anterior extension of the paired or dorsal mesoblast appears


Fig. 58. - Late Gastrula Stage of the Newt (Triton). [After 0. Hertioig.]

A. Median vertical longitudinal section. B. Horizontal section through the same.

a. archenteron ; bp. blastopore ; d.l. dorsal lip of blastopore ; ep. epiblast ; hy. dorsal or axial hypoblast ; l.l. lateral lip of blastopore ; m. dorsal mesoblast ; v l. ventral lip of blastopore ; v.m. unpaired ventral mesoblast ; y.hy. yolk-hypoblast.

to occur at the expense of the hypoblast, in a similar manner to that described for Amphioxus (fig. 59). Hertwig describes the dorsal layer as arising from the “ Chorda-entoblast - (axial or notochordal hypoblast), and the ventral from the “Darm-entoblast - (digestive or gut hypoblast).

Fig. 59. - Transverse Section of the Dorsal Portion of an Embryo Newt (Triton). [After 0. Hertwig .]

a. mesenteron; ax.hy. axial hypoblast in process of forming the notochord ; b.c. coelom (body -cavity) ; ep. epiblast ; hy. digestive hypoblast ; n.p. neural plate ; so.m. somatic mesoblast ; sp.m. splanchnic mesoblast.


In the Newt, and all the higher Chordata, as in Amphioxus, the axial hypoblast or notochord is in direct contact with the neural epiblast, consequently the dorsal mesoblast is distinctly paired. There is a ventral growth of unpaired mesoblast from the lower lip of the blastopore (fig. 58, A, v, m). This occurs at the spot where the epiblast and hypoblast pass into each other, and it is difficult to say which layer has the larger share in its formation ; if either, it is perhaps the epiblast.

The formation of the mesoblast in the Lamprey is, according to Calberla, practically identical with that in the Newt; in some


Fig. 6o. - Late Gastrula Stage of Lamprey. [After Scott.]

A. Median longitudinal vertical section. B. Section to one side of A.

The relation of the hypoblast to the mesoblast is more clearly seen in fig. 61. a. archenteron (mesenteron) ; bp. blastopore ; ep. epiblast ; hy. hypoblast of mesenteron, axial hypoblast in A. ; m. paired mesoblast ; m' ventral unpaired mesoblast ; y.liy. yolk-hypoblast.


respects it is simpler than in the latter, owing to less food- yolk being present in the ovum. The position of the paired mesoblast is clearly shown in figs. 60 and 6 1. The two latero-dorsal sheets extend from the lip of the blastopore some distance forwards, but


Fig. 6i. - Transverse Sections through the Upper Portions of Two Embryo Lampreys (Petromyzon planeri). [After Calberla .]


A. Same stage as fig. 63. B. Later stage.

a. archenteron ; ax.hy. axial (notochordal) hypoblast ; ep. epiblast ; hy. (digestive) hypoblast ; m. mesoblast ; n.k. neural keel ; n.p. neural plate ; y.h. yolkhypoblast.


they have not yet acquired any lateral or rather ventral extension ; dorsally they are separated from one another by the axial hypoblast (figs. 60, A, and 6 1, a).

According to Scott, a single layer of mesoblast (fig. 60, m') surrounds the lateral and ventral surface of the yolk-hypoblast from which it is derived ; he also states that the paired mesoblast grows forward from the blastopore, and that it does not exhibit any intimate relation with the axial hypoblast.

In his recently published paper, Shipley states that in Petromyzon fluviatilis the first formation of the mesoblastic plates appears to take place by a differentiation of the hypoblastic yolkcells in situ , and not from invaginated cells. The subsequent downward growth is brought about by the cells proliferating along the free ventral edge of the mesoblast ; these cells then growing ventralwards push their way between the yolk-cells and the epiblast.


Fig. 62. - Origin of Mesoblast in the Frog. [After 0. Hertwig .]


A. Median longitudinal vertical (sagittal) section through a gastrula with a wide blastopore. B. Enlarged view of a portion of the same. C. Horizontal (frontal) section through a stage similar to that of A. D. Lateral lip of a corresponding stage. E. Horizontal section through a nearly closed blastopore.

F. Section through the anterior lip of a closed blastopore.

a. archenteron ; ax.liy. axial hypoblast; be. blastocoel ; bl. blastopore; d.l. dor~al, l.l. lateral, v.l. ventral lip of blastopore; ep. epiblast ; hy. hypoblast; m. lateral mesoblast ; v.m. ventral mesoblast; y. Ivy. yolk hypoblast.


In the Frog the mesoblast has a fundamentally similar origin to that above described, but the invagination of the mesoblast is less marked. The greater portion of the mesoblast is apparently derived by the metamorphosis of the small cells of the yolk-hypoblast in situ (figs. 24 and 62) ; the result being that there is very early a sheath of mesoblast, one or more cells thick, below the epiblast. The mesoblast is only interrupted along the median dorsal line. The explanation of figs. 24 and 62 sufficiently illustrate tlie character of the mesoblast of the Frog on its first appearance.

Mitsukuri and Ishikawa have very recently shown that in the Turtle (Trionyx) the formation of the mesoblast closely recalls the same process in the Newt. Fig. 63, which represents a transverse section through the hind-portion of the head, demonstrates the paired mesoblast as arising by proliferation from the hypoblast at the spot where the digestive hypoblast is contiguous with the


Fig. 63. - Mesoblast of Trtonyx. [ After Mitsukuri and Ishikawa .]

A. Transverse section through the head region before the closure of the neural groove. B-D. Portions of successive sections of the same embryo.

am. amnion; ax.hy. axial hypoblast; Ip. a. epiblastic and hy.a. hypoblastic layer of amnion ; liy. hypoblast ; m. mesoblast ; nc. neural canal ; ncli. notochord.

axial or notochordal hypoblast. In this case, as in so many other instances, the proliferation may be regarded as a degenerate form of invagination.

Behind the blastopore the mesoblast arises, as in Amphibia, as an unpaired mass, and in this region there is a fusion of the three germinal layers, thus forming a primitive streak.

The formation of the mesoblast in the Lizard (fig. 64) is intermediate between that which occurs in the Turtle and the Fowl. The paired mesoblast has much the same origin as that to be shortly described for the Fowl. It arises posteriorly from the walls of the blastopore as a pair of lateral sheets, which are free for the greater portion of their extent, but are fused in the median line of the posterior region of the embryo with the axial hypoblast. Anteriorly the mesoblast is derived from branched cells, which are budded off partly from the axial, and partly from the lateral hypoblast (fig. 64, m).


Fig. 64. - Transverse Section through a Portion of the Blastoderm of a Lizard (Lacerta Muralis). [Ajter Weldon.]

The section illustrates the double origin of the mesoblast in the embryonic region, i.e., in front of the primitive s' reak.

a. mesenteron ; ax.hy. axial hypoblast, which is about to develop into the notochord ; ep. epiblast ; hy. hypoblast ; m. mesoblast, partly derived from the axial, and partly from the permanent.hypoblast ; n.f. neural fold ; n.g. neural groove.


The origin of the mesoblast has been very carefully studied in Birds. One portion of the mesoblast arises as a pair of lateral plates by the proliferation of the epiblast along the line of the


Fig. 65. - Transverse Section through the Anterior End of the Primitive Streak of a Fowl -s Blastoderm about the Age of Fig. 34. [ From Balfour.]

Showing the rounded mesoblast cells arising from the primitive streak and the stellate cells of hypoblastic origin.

ep. epiblast ; hy. hypoblast ; m. mesoblast ; p.v. primitive groove ; yh. yolk of germinal wall.

primitive streak (fig. 65). Balfour even says that during this period many sections through the primitive streak give an impression of the mesoblast being involuted along the lips of a groove. A second portion of the mesoblast is that which gives rise to the lateral plates of mesoblast in the head and trunk of the embryo. This is formed of stellate cells, which are at first readily distinguishable from the rounded cells of the former class ; they arise from the hypoblast mainly on each side of the median line, and especially in the region in front of the primitive streak ; in other words, in the embryonic region. They are continuous behind with the lateral wings of mesoblast which grow out from the primitive streak, and on their inner side are also at first continuous with the cells which form the notochord.

The third portion of the mesoblast is derived partly from those cells of the lower-layer cells which do not form the permanent hypoblast, and which are scattered between that layer and the epiblast (figs. 30-34), and partly from the germinal wall, or that ridge of cells, nuclei, and yolk-granules which in the early stages


Fig. 66. - Section through the Germinal Ridge of a Fowl -s Blastoderm. [After Kcllmann.]

a. archenteron ; ep. epiblast ; hy. hypoblast ; m. mesoblast cells (mesamceboids or “ Poreuten -) which have been derived from the primitive hypoblast cells of the germinal ridge ; y. yolk ; y . yolk-spheres ingested by the primitive hypoblast.

of incubation forms the marginal boundary of the lower -layer cells or primitive hypoblast (figs. 65, 66). The large primitive hypoblast cells of the germinal wall are undoubtedly nutritive in function, and ingest the underlying yolk. By cell-division they give origin to amoeboid wandering cells (fig. 66, m), which are stated by Kollmann to form the primitive vascular system, the blood, and also the connective tissue. In either case, the cells have the same morphological value since they are derived from lowerlayer cells before the hypoblast proper is differentiated.

While the paired mesoblast referred to above is clearly mesothelial in character, the mesoblast which arises from the lowerlayer cells and the germinal wall appears to be mesenchymatous in nature.

The development of the mesoblast in the Mole (Talpa) (fig. 67) has been shown by Heape to agree very closely with that described above for Birds. Posteriorly the mesoblast arises where the epiblast and hypoblast are fused at the primitive streak, and clearly owes its existence to both. In the region in front of the primitive streak the mesoblast is proliferated from the hypoblast as two lateral masses which posteriorly unite with the abovementioned mesoblast. There also appears to be an actual continuity between the developing notochord and the dorsal portion of the paired mesoblast.

There is some diversity of opinion amongst other investigators concerning the origin of the mesoblast amongst Mammals. It may be concluded that the Mole, being an Insectivore, would probably not have a very specialised development for


Fig. 67. - Sections through the Blastoderm of a Mole (Talpa). [After Heape.\

A. Longitudinal section through the middle line of part of an embryonic area in which the primitive streak has commenced to form ; the blastoderm is perforated in front of the primitive streak. B. Transverse section through the middle of a well-developed primitive streak; the epiblast and mesoblast are fused, but the hypoblast is distinct; the mesoblast here extends beyond the embryonic area. C. Same as B, but through the hind-knob of the primitive streak. All the layers are fused in the embryonic area, but are distinct beyond.

bp. blastopore ; ep. epiblast ; hy. hypoblast ; m. mesoblast ; p.s/c. primitive streak.

a Mammal, and, for the present, the above statement may be regarded as holding good for Mammalia generally.

When the embryology of the Prototheria (Ornithodelphia) is investigated, it will doubtless be found to resemble that of the Lizard in many points, and will demon* strate that any peculiarities in the development of Mammals is due first to the presence, and secondly to the subsequent loss, of food-yolk.

Although in most Vertebrates the mesothelial mesoblast is at first solid, it very shortly splits into two layers, a peripheral or somatopleur, and a visceral or splanchnopleur (figs. 59, 71). The pleuro-peritoneal cavity or coelom thus produced is strictly homologous with the persistent body-cavity of such forms as have hollow archenteric diverticula.

It is evident from the foregoing summary that the derivation of the true bodycavity or coelom from archenteric diverticula occurs in one or more examples of nearly all the main groups of the animal kingdom. In the majority of cases it occurs in generalised, or, geologically speaking, in ancient types. It may then be safely concluded that this is the primitive method of the formation of the coelom. This statement does not preclude the possibility of interstitial spaces or cavities occurring, as in Platyhelmintlies, Arthropoda and Mollusca ; but these, not being lined by an epithelium derived from the archenteron, should always be distinguished as pseudoccelous or archiccelous cavities, as opposed to a true body-cavity. It is known that mesodermal (mesenchymatous) cells bounding a pseudoccel (archiccel), or cavities derived therefrom, may sometimes become flattened and form an endothelium, t - There can be no doubt that the lateral sheets of mesoblast of Vertebrates with telolecithal ova are identical with the mesoblastic somites of Amphioxus, and the latter again with the archenteric diverticula of many Invertebrates.

A very instructive series can be traced from such an alecithal ovum as that of Amphioxus through the Lamprey, Newt, Frog, Turtle, and Lizard, to the extreme telolecithal type of the Bird. The lateral proliferation of the hypoblast in the Lizard and Fowl (figs. 64, 65) to form the mesoblast is possibly a secondary process. Throughout this series the axial hypoblast takes its share in formation of the paired mesoblast along with what has been spoken of as the digestive hypoblast. %

The primitive-streak mesoblast, as it is termed, is the equivalent of the mesoblast which arises from the lips of the blastopore ; as, for example, in the Newt. A reference to the section dealing with telolecithal gastrulation and the nature of the primitive streak will render further comment needless.

Allusion has previously been made to the origin of certain indifferent mesoblast cells from the primitive hypoblast, which appear to differ in character from the former, and which have been regarded as being mesenchymatous in nature.

Summary

The following is a brief resume of the mesoblastic elements of the Metazoa.

Mesamoeboids arise, apparently indiscriminately, from the endoderm (hypoblast) of larval and adult Sponges, and from the same layer in Coelenterates. The cells which migrate from the ectoderm into the gelatinous tissue in the latter group are practically epiblastic mesoderm. In most of the Coelenterates arch enteric diverticula are found, which never become separated from the alimentary canal.

In the Echinoderms, mesamoeboids arise in the blastula stage, chiefly, if not entirely, from the incipient hypoblast ; and after the formation of the gastrula, archenteric diverticula arise, which become completely shut off to form the body- cavity of the adult.

The mesamoeboids of the Platyhelminths are derived, in some cases, at all events, from both layers of the gastrula.

The exact nature of the mesoblast of Molluscs has not yet been satisfactorily demonstrated.

The Arthropods and their ancestors, the Segmented Worms, possess an enterocoelous body-cavity, although, in the great majority of cases, its method of development masks its real nature. The presence of mesenchymatous mesoblast in these groups has been questioned.


Lastly, in Vertebrates the mesothelial mesoblast is extremely well developed ; according to some investigators, mesenchyme is also present.

It is worthy of note that mesenchyme is certainly phylogenetically older than mesothelinm, and that those requirements which caused it to first develop may have continually recurred ; so that whereas all the tissues or organs derived from mesothelium are, to a certain extent, homologous, those composed of mesenchyme may not necessarily be so.

In this connection it is interesting to note that, according to Sedgwick, at an early stage in the development of Vertebrates, most of the connective tissues of the wall of the body and gut are derived by a process of growth outwards of cells from the epithelium of the body-cavity. The same, he believes, holds good for the connective tissue and blood-vessels of the Wolffian body.

Ccelomic cavities. -Sedgwick has very recently drawn attention to the history of the cavities enclosed by mesothelial mesoblast. He finds from his researches on Peripatus that it is probable that throughout the Arthropoda the cavity of the body and all the vascular spaces are pseudoccelous. The lumen of the generative organs is in all cases coelomic, as is also the nephridial apparatus of Peripatus. The excretory organs of other Arthropoda require re-investigation.

In Mollusca the pericardium, nephridia and possibly the ducts of the generative organs are coelomic. The vascular system and all the lacunae in the body are pseudoccelous.

In the Chaetopoda and Chordata the cavity of the body is entirely coelomic, and from its walls are derived the nephridia and the generative organs. The pseudocoel (archicoel) is only represented in the adult by the complicated system of vascular channels.


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Haddon 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B


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