12 GENERAL DEVELOPMENT OF THE EMBRYO. 



already been described in Balfour's Memoir on Peripatus 

 (loc. cit.), is found most commonly at about the middle or 

 June in England. 



The embryo now grows considerably in length (fig. 23), the 

 blastopore presenting a corresponding elongation, and the meso- 

 derm, 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, each of which contains 

 a cavity, part of the future body cavity. The first somite to 

 appear is the anterior, and then successively backwards. 



Stage B (fig. 25). — The blastopore now divides into two 

 parts (figs. 24 and 25) 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 dis- 

 tance 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 (fig. 25) . 



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 prseoral lobes. 



Stage C (figs. 26 and 27). — The hind end of the body now 

 becomes curved ventrally (figs. 26 and 27). The curve is pro- 

 duced 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 (fig. 28). 



Stages B and C are found in July and August at the Cape. 



Stage D (figs. 28 and 29) . — The spiral stage is characterised by 

 the appearance of the appendages and of the lip-fold which 

 encloses the jaws in the adult, and of the eyes. 



