i86 



THE ANCESTRY OF VERTEBRATES 



side, is seen to occur during the development of Annelids. 

 The primary ectoderm is represented here by the three 

 quartets of micromeres, the primary endoderm by the re- 

 maining quartet of macromejes which, however, merit this 

 name in yolk-laden eggs only Let us first leave the case of 

 yolk-richness out of consideration and consider the develop- 

 ment of an egg with less or hardly any yolk, in which the 

 macromeres scarcely, if at all, surpass the micromeres in size. 

 While the endoderm which remains after the production 

 of the three quartets of ectomeres lies originally diame- 

 trically opposite the animal pole, we find the mouth, which 

 is directly to be traced back to the blastopore, in the tro- 

 chophora lying on the future ventral side, just under the 

 prototroch which forms the bbrder of the apical plate. As 

 1 have discussed in my article on the development of 

 Scoloplos armiger (1916 a), the displacement is to be 

 ascribed to three factors. 



Fig. 36. Diagrammatic representation of the behaviour of the 

 blastopore, a, b, c, d in polychaetous Annelids, e, f, g in 

 Chordates. 



bl. blastopore, d gut, ent. entoderm, h.pl cerebral plate, m. 

 mouth, m pi. medullary plate, neur. neurotroch,pr. prototroch. 



In the first place we observe a moving of the whole 

 endoderm area to the ventral side (fig. 36 b), a result of 

 the active multiplication and extension of the ectoderm 

 cells at the rear side, i.e. mainly the rf-quadrant of the 

 egg, whereas the cells of the anterior side, the Z)-quadrant, 

 are backward in development. This causes the endoderm- 

 area to move to the ventral side to such an extent that 



