DEVELOPMENT OF SPELERPES BILINEATUS 235 



separate and appear to lie about an uninvaginated yolk mass. 

 These experiments substantiate the results obtained in locating 

 the embryo on the upper hemisphere. In the next section I 

 shall attempt to show why the divergent posterior neural folds 

 cannot substantiate the theory of concrescence. 



Concrescence and convergence 



Since His put forth the view of the formation of the animal 

 body by the approximation of two separate lateral halves, much 

 study has been given to determining whether or not this view is 

 true. The tide of opinion has surged first one way and then the 

 other. Patterson ('07), working with chick embryos, has recently 

 demonstrated that, by taking a sufficiently early stage, there is 

 a concrescence of the blastopore lips, which fuse to form the 

 primitive streak. In the Amphibians, after the blastopore 

 has become slit-like, a concrescence by fusion of its lips has been 

 noted. This fusion forms the primitive streak and in this respect 

 is like the chick. I see no objection to this view of concrescence. 

 Another view, maintained by Roux, Hertwig and others, con- 

 sidered that the entire embryo was laid down in two halves in a 

 ring about the equator. They point to spina bifida embryos as 

 proof. Now, there can be little doubt but that much of the mate- 

 rial out of which the embryo of many Anura is formed, lies in the 

 equatorial regions at one stage of development. It by no means 

 follows, however, that the two halves of the embryo gradually 

 approach each other, beginning at the head, and fuse, leaving 

 a visible sign of this fusion in the neural groove, as Hertwig 

 and others maintain. I shall return to this. 



Morgan ('94, '97, '03), and others, while not admitting concres- 

 cence by 'apposition,' believe that part of the embryo, at any 

 rate, is formed by "concrescence from before backward at the 

 dorsal lip of the blastopore," that is, the material to form the 

 dorsal lip and embryo lies in the lateral lips of the blastopore and 

 moves along the parallels of the egg towards the median line, 

 there to fuse, thus causing the dorsal lip to appear to advance. 

 Its material once in place, remains stationary. 



