137 
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 macromeres 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 border of the apical plate. As 
| 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 
lastopo ‚c,d in polychaetous Annelids, e, f, 2 in 
Chordates. 
bl. blastopore, d gut, ent. entoderm, h. pl. cerebral plate, m. 
mouth, m. pl. medullary plate, zeur. 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 d-quadrant of the 
egg, whereas the cells of the anterior side, the b-quadrant, 
are backward in development. This causes the endoderm- 
area to move to the ventral side to such an extent that 
