DEVELOPMENT OF SPELERPES BILINEATUS 201 



In the first case, we may suppose that they pull up and roll 

 around the end of the archenteron (figs. 30-32) and so come to 

 lie in its roof; or, in the second case, we may suppose that they 

 do not roll around the end, but maintain their position in the 

 floor of the archenteron and are carried forward by the extension 

 of the archenteron to their final position at its anterior end, that 

 is, from h to b', fig. 35a. The roof of the archenteron may be 

 supposed to be formed by a process of splitting and rearrange- 

 ment of the floor cells of the blastocoele, part of which move in 

 the direction of the large arrows in fig. 35a. Others, especially 

 those near b' and c, may move in the direction c to c' and come 

 to he next the ventral ectoderm. Some of the cells beneath the 

 archenteric split may even come to form part of the anterior 

 roof of the archenteron. Those yolk cells forming the outer layer 

 of the ventral hemisphere of the blastula (fig. 35a, c-c'), then, 

 would come to form much of the floor of the archenteron, since 

 definite proof is given below that these cells actively invaginate. 

 If we could superimpose the roof of the completed archenteron as 

 shown in a vertical section, onto a similar section of the blastula, 

 it would appear much as in fig. 35a. It is obvious that if the 

 cells at b move to b', the cells between c and b would form much 

 of the floor. If^ however, the first case suggested is true, then 

 the roof of the archenteron has a mixed origin, part of its cells 

 being those which earlier formed the floor of the blastula and 

 part external yolk cells. Its floor, on the other hand, is formed 

 in a different way from that suggested for the second case, its 

 posterior part being formed from external cells near c, and the 

 remainder being derived by a process of splitting from the yolk 

 ceUs of the interior. At present, conclusive evidence, bearing on 

 these suggestions, is wanting. 



The mode of origin of the archenteric roof has been a source of 

 much discussion, due partly to actual differences among the Am- 

 phibia, and partly to differences of opinion as to the value of the 

 evidence opposed. Thus, some beheve that the presence of pig- 

 ment is proper evidence, while others deny this. 



For the most part, it is generally conceded that some part of 

 the archenteric roof is formed by an invagination of those cells 



