356 



Mr. A. Sedgwick. 



LMay 21, 



An opacity appears at the hind end of the blastopore. This opacity 

 is the primitive streak. It appears to be due to the active prolifera- 

 tion of some cells, which cannot be definitely assigned either to the 

 ectoderm or the endoderm, at the hind end of the blastopore. This 

 stage, which has already been described as stage A in Balfour's 

 Memoir on Peripatus (" Quarterly Journal of Microscopical Science," 

 vol. 23, Plate 20, fig. 34), is found most commonly at about the 

 middle of June. 



The embryo now grows considerably in length, the blastopore pre- 

 senting a corresponding elongation, and the mesoderm, which arises 

 from the proliferation of the undifferentiated cells of the primitive 

 streak, grows forward as two ventro -lateral bands, one on each side 

 of the blastopore. 



The mesodermal bands next divide by transverse division from 

 before backwards into somites, which contain a cavity, part of the 

 future body cavity. The first somite to appear is the anterior, and 

 then successively backwards. 



The blastopore now divides into two parts by the obliteration of its 

 median portion into an anterior part which becomes the mouth of the 

 adult, and a posterior part which is at first placed at some little 

 distance from the hind end of the embryo and gives rise to the anus 

 of the adult.* 



The primitive streak still persists and extends from the hind end of 

 the blastopore to the hind end of the embryo. It is now marked by 

 a groove — the primitive groove. 



The anterior pair of somites have shifted forward to quite the 

 anterior end of the body ; they give rise to the mesoderm and body 

 cavity of the praeoral lobes. 



This stage has already been described and figured as stage D in 

 Balfour's memoir (op. tit., Plate 20, fig. 37). 



The hind end of the body now becomes curved ventrally. The 

 beginning of this curve is shown in the figure of stage E in Balfour's 

 memoir (Plate 20, fig. 38). The curve is produced by the growth of 

 the hind end of the body. As this growth proceeds the curve becomes 

 more marked, and assumes a spiral form, that is to say, the hind end 

 of the body is spirally coiled, the coil being applied to the ventral face 

 of the anterior part of the body. (Vide Moseley's figure in the " Phil. 

 Trans.," vol. 164, Plate 75, fig. 1.) 



* There is no structure in the embryos of Peripatus capensis corresponding to 

 the amnion or placenta described by v. Kennel in the West Indian species (Semper's 

 '•' Arbeiten," Bd. 7). The early stages of that species are obviously difficult to 

 follow, and have not been entirely made out by that observer. I think it, therefore, 

 better not to offer any criticisms on his statements, particularly those with regard 

 to the amnion, until his observations have been either extended or confirmed. — 

 (May 25, 1885.) 



