703 



dition of new cells coming up from the sides. These new cells that 

 come up beneath the surface to take their position on the dorsal wall 

 of the archenteron are the cells that have formed the outer wall along 

 the blastoporic invagination. 



In addition to the cells that have pulled in around the lips of 

 the blastopore, it is not improbable that as the lateral lips approach 

 the median line, a certain amount of actual overrolling of the edge of 

 the blastopore takes place, so that some cells lying just outside of 

 the crescent will also participate in the formation of the dorsal wall 

 of the archenteron. Jordan (4) has watched in the living egg of the 

 frog the cells just outside of the blastopore and has seen them actu- 

 ally turning into the archenteron. Since, however, the crescent of the 

 blastopore forms in the midst of rather large cells it does not seem 

 best to speak of these cells as ectoderm. 



Concrescence. 

 The conclusion reached as to the method of formation of the em- 

 bryo is that the embryo forms by a process of concrescence. Not con- 

 crescence by apposition, but concrescence by fusion from before back- 

 ward at the dorsal lip of the blastopore. That the process is not 

 simply apposition seems to me probable from this fact. In those cases 

 where the yolk-plug remains exposed in the mid-dorsal line, we find 

 in nearly all cases a tendency for the median organs lying behind the 

 yolk-plug to be doubled. Although in these cases the blastoporic rim 

 has met and fused behind the yolk-plug by apposition, yet the 

 fusion is not so perfect as when the material has met in the dorsal 

 lip. In nearly all these cases when the tail appears it develops from 

 a pair of tail knobs, as Hertwig has figured for the spina bifida em- 

 bryos. The whole tail as it elongates shows more or less a double 

 structure. In a few cases two tails form right and left. It seems at 

 first sight remarkable, that injury to the dorsal lip of the blastopore 

 should produce double tailed embryos, but from the evidence given above, 

 it seems most certain that the double tail is not due to a splitting 

 of a growing region (dorsal lip), but to an imperfect union of parts 

 behind the yolk-plug. It is possible that the cause of this imperfect 

 union may not be altogether on account of the apposition, but because 

 of the delay in the fusion of the two sides behind the yolk-plug. Even 

 if this be true however, it is hard to see why the tail knobs of em- 

 bryonic tissue should not perfectly fuse. 



