GASTRULATION AND EARLIEST DEVELOPMENT 



187 



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 ifig. 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 ihe third quartet 

 it is only the cells of the anterior two quadrants, 3a and 3b, 

 and of the second quartet only 2a, 2b 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 



-■.stonrv. 



.1 — neurtr. 



Fig. 37. Representation of the fate of the blastopore in Annelids. 

 Dotted are the cells of the S^d quartet of micromeres 

 (III), surrounding the blastopore, blank those of the 

 2nd quartet (II). 

 an. anus, near. tr. neurotroch, stom. stomodaeum. 



mouth comes to lie just underneath the prototroch (fig SecQ. 

 The cells surrounding the posterior half of the original 

 wide blastopore border, being the descendents of 3c an M, 

 form toj^ether the neuiotroch, a ciliated band ofvacuolized 

 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 \\ay the neurotroch has 

 originated (fig 37). 



