50 THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 



Fig. 51. Section passing through the middle of the primitive streak which has begun 

 to double over the ventral surface of the embryo and become independent of the trophic 

 vesicle. The embryonic ectoderm is coextensive with the mesoderm which, at this stage, is 

 rapidly breaking up into hollow somites. Beyond the dorsal edge of the somites (on the 

 lower side of the figure) the embryonic ectoderm passes sharply into the trophic ectoderm 

 which can be easily distinguished in all sections by the character of its cells. Beyond the 

 frontal or ventral edge of the somites the thickened lateral ectoderm passes into the low 

 ectoderm which now occupies the ventral surface of the embryo proper. The primitive groove 

 occupies the median ventral line of the primitive streak, its position being apparently reversed 

 from that of previous stages in consequence of the recurvature of the free growing end of 

 the embryo. Thus in previous stages the primitive groove faced outwards, now it faces inwards. 

 The section may have been slightly oblique since two somites are seen on the left side of 

 the figure, namely, somites 3 and 4, while only somite 4 is seen in section on the right 

 of the figure. The primitive streak as here figured is essentially identical with that of 

 P. capensis as figured by Sedgwick in his Stage C. It is quite solid and consists of a 

 mass of proliferating cells in which may be distinguished an outer portion or mes-ectoderm 

 and an inner core or nies-endoderm, the latter proceeding from the former. The chorion is 

 shown in the figure. 



Fig. 52. Section passing through the plane in which the outgrowing caudal region unites 

 with the main body of the embryo. This region lies posteriorly to the preceding with relation 

 to the cephalic end of the embryo, but a little reflection will show that the superincumbent 

 caudal portion of the embryo is morphologically anterior to the free primitive streak of Fig. 51. 

 We have in fact got beyond the primitive streak. The somite to the right of the figure 

 is the anterior portion of the fifth somite (counted, of course, from the head) ; that to the 

 left is the posterior portion of the fourth somite. This obliquity may be merely due to an 

 accident of cutting or it may be parti}' due to a slight inequality in growth. The irregular 

 cavities in the mes-endodermic mass are portions of the trophic cavity and their appearance 

 is due to the fact, that, so far as the caudal portion of the embryo is concerned, the section 

 cuts the caudal mesoderm tangentially. 



Fig. 53. Section passing through the tail-swellings with nascent somites. This section 

 is posterior to the preceding, that is to say, nearer the posterior end of the trophic vesicle, 

 but it is easy to understand that the nascent caudal somites are approaching the trunk- 

 somites. The conditions here portrayed are at first rather puzzling, but they are due to the 

 fact mentioned in the text that the flexure of the embryo occurs in situ. The trunk-somite 

 to the right of the figure is the anterior tip of the sixth somite ; that to the left is the 

 middle of the fifth. Behind this region the tail-swellings gradually flatten out until finally 

 the embryonic region is passed over and we find as before merely trophic ectoderm and 

 endoderm. 



Fig. 54. Section passing tangentially through the posterior portion of the "head-swellings." 

 It shows the transition from the thin ectoderm of the ventral abdominal surface into the 

 thickened ectoderm of the cranial groove and cephalic region generally and also the entrance 

 to the stomodoeum. On the right of the figure, somites 3 and 4 are seen in section, and 

 to the left somites 2 and 3. This section passes through the posterior margin of the cephalic 

 region which forms the anterior limit of the crescentic groove shown in Figs. 27 and 28. 



