The Development of Ischnochiton. 619 



divides into four cells and these move rapidly into the gap. The 

 anterior half of the archenteron therefore consists of a roof of macro- 

 meres with side walls composed of fourth quartette cells in the antero- 

 posterior line and at positions 90" distant in the right and left 

 quadrants, while between them lie the products of the third quartette 

 stomatoblasts. 



Now to the formation of the posterior wall. As I have said, the 

 stomatoblasts of the third quartette in quadrants G and D in their 

 first division give rise to small cells that are relatively unimportant, 

 while the remaining cells are much larger and lying by the side of 

 the mesoblast accompany it deep into .the egg. As the mesoblast 

 sinks in the movement of the stomatoblasts is in an opposite direction, 

 by which they come to lie almost wholly on the surface of the embryo 

 as in Fig. 39, at the same time approaching each other along the 

 median ventral line. They now divide forming five cells in each 

 quadrant and continuing their movement inward become almost lost 

 to view. Sections show that at about this point their invagination 

 ceases and for a considerable period they may be seen on the 

 posterior lip of the blastopore in connection with the lower surface 

 of the mesoblast. 



Sections or entire embryos at this stage will show that even 

 though the direct inward movement of the mesoblast is checked there 

 is a shifting of its cells by which they come to lie with their anterior 

 surfaces in line with the inner surface of the archenteron, while the 

 posterior surfaces extend far backward and also downward into the 

 ectoderm of the posterior half of the embryo. A glance at Fig. 37 

 may make this point clear. Were all the fourth quartette cells alike, 

 4d (mesoblast) would be similar to 4 a on the right of the figure. In 

 that case, in the invagination the macromeres would slip past the 

 fourth quartette cells as in the right half of the figure. But 4d has 

 become modified: it has grown larger than its homologues in the 

 other quadrants and, of the greatest importance perhaps, it is globular 

 in form. Owing to this latter peculiarity , the macromeres press in 

 above this cell and it becomes pushed outward and downward. Thus 

 a distinct bulging occurs on the lower posterior side of the embryo 

 and the ectoderm in this region becomes comparatively thin, much 

 thinner than at any point on the anterior side of the embryo, and 

 this character is apparently correlated with more rapid cell division 

 in this region by which in a relatively short time the ectoderm cells 

 of the posterior side of the egg double those of the anterior. 



