238 HUBERT DANA GOODALE 



the neck, then the sides of the body and finally reach the material 

 for the tail. All we have to do is to swing the two halves together 

 by the tail ends and the embryo is formed. Even the seam, along 

 which the two halves unite, is there, i.e., the neural groove. 



According to the theory of convergence, the process is more 

 complex . The material out of which the embryo is to form is not yet 

 differentiated, or if differentiated, is still incompletely separated. 

 The material for the tail must be spread out along the entire 

 equatorial band, for example. This follows from the fact that 

 a mark made in the dorsal lip and one in the lateral lip, both reach 

 the tail. The material for the neck region in the frog is not so 

 widely scattered but lies close to the point of origin of the blasto- 

 pore. As a consequence, it follows that the most complex (or 

 least differentiated) part of the blastopore lip is the dorsal, while 

 the simplest part is 90° from the median line of the embrj^o, since 

 this part will form only tail and ventral ectoderm. I have not 

 gone into the part played by the ventral lip since it is obvious 

 from my point of view that it furnishes only ventral ectoderm. 

 I am, moreover, speaking of the superficial parts only, ' I cannot 

 agree, then, with Moszkowski, ('02), who maintains that the 

 formation of the blastopore and the differentiation of the embryo 

 are independent. 



Before leaving this question, I wish to point out that spina 

 bifida embryos and their extreme form, ring embryos, can be 

 explained on this view as well as the view of concrescence. The 

 material is already laid down in a ring about the equator. Any- 

 thing that prevents the movement of this material, as happens 

 when the yolk is prevented from invaginating, forces it to differ- 

 entiate in situ. In any case, half structures will be formed around 

 the yolk plug, since only half of the material is already present. The 

 experiments of Roux and others in killing one of the first two blas- 

 tomeres, show, that as long as the material for one-half the em- 

 bryo is forced to keep its primary arrangement as a half-structure, 

 only half an embryo will develop. It is only when the half is 

 permitted to rearrange itself, as Morgan showed by inverting 

 an egg after killing one blastopore, that a whole embryo can de- 

 velop (compare also, MacClendon '09). Hence we should not 



