Fig. 10. Diagrammatic representation of the behaviour of the blastopore, see text. 
a, b, c, d in polychaete annelids, e, f, g in chordates. bl. blastopore, d. gut, 
ent. entoderm, A. pl. cerebral plate, m. mouth, m. pl. medullary plate, neur, 
neurotroch, pr. prototroch. ~ 
the 6-quadrant, are backward in development. This causes the entoderm 
area to wander to the ventral side to such an extent, that no longer 
its centre but its hind border is found opposite the animal pole. In 
this region afterwards the anus is formed. 
Secondly the blastopore does not close concentrically, but excen- 
trically in a forward direction (Fig. 105), be it with or without 
concrescence of the lateral borders. This depends on the relative 
speed with which either the lateral borders or the hind border move 
forward over the entoderm, and this again depends on the way in 
which the descendents of 2d, the so-called somatic plate, spread over 
the left and right side and over the posterior end of the embryo. 
Evidently conerescence here seems to be the rule and at the suture, 
where left and right blastopore borders have met, the neurotroch arises. 
In the third place the foundation. of the stomodaeum here does 
not any longer surround the blastopore as a ring of uniform breadth, 
as in Protaxonia, but lies more in the way of a crescent round the 
anterior border. For of the third quartet it is only the cells of the 
anterior two quadrants, 3a and 35, of the second quartet only 2a—2c, 
which participate in the formation of the stomodaeum. After the 
sinking in of this erescentic rudiment to the formation of the 
stomodaeum-tube, which arises outside the final, narrowed blastopore, 
the mouth comes to lie just underneath the prototroch (Fig. 10d). 
We shall see now what we find of these phenomena in the frog 
