138 
no longer its centre but its hind border is found opposite 
the animal pole. In this region the anus is afterwards formed. 
Secondly the blastopore does not close concentrically 
but eccentrically in a forward direction (fig. 36 c), be it 
with or without concrescence of the lateral borders. Evi- 
dently concrescence here occurs as a rule and the neu- 
rotroch arises at the suture where left and right blastopore 
borders have met. 
In the third place the foundation of the stomodaeum no 
longer surrounds here the blastopore as a ring of uniform 
breadth, as in Protaxonia, but lies more in the form of a 
crescent round the anterior border, for of the third quartet 
it is only the cells of the anterior two quadrants, 3a and 
3b, andof the second quartet only 24 and 2c which participate 
in the formation of the stomodaeum. After the sinking in 
of this crescentic rudiment to form the stomodaeum-tube 
which arises outside the final narrowed blastopore, the 
Fig. 37. Representation of the fate of the blastopore in Annelids. 
Dotted are the cells of the 3rd quartet of micromeres 
(III), surrounding the blastopore, blank those of the 
2nd quartet (II). 
an. anus, neur. fr. neurotroch, stom. stomodaeum. 
mouth comes to lie just underneath the prototroch (fig 36d). 
The cells surrounding the posterior half of the original 
wide blastopore border, being the descendents of 3c an 3d, 
form together the neurotroch, a ciliated band of vacuolized 
cells along the median ventral line. In the radially sym- 
metrical ancestors of the Annelids, where no doubt the 
stomodaeum had a radial origin, these cells probably have 
participated at its formation. We can imagine that this 
original stomodaeum has partly closed by coalescence of 
its lateral borders and that in this way the neurotroch has 
originated (fig 37). 
