REGULATION IN ANURAN EMBRYOS 181 



For in some of them the organs develop in very unequal degree 

 in the two lateral blastopore lips. This asymmetry is extreme 

 in the embryo pictured by Hertwig in figures 15 and 16 of pi. 

 XVI and figure 27 of pi. XVIII. In this individual development 

 had gone so far that eyes and suckers had been formed. Never- 

 theless, nearly the whole of the dorsal surface was occupied by 

 an exposed yolk area. The special point of interest is that one 

 pf the lateral blastopore lips was found to be not organized at 

 all. The opposite lip, on the contrary, contained a small neural 

 tube and notochord, which anteriorly were continued into the 

 corresponding structures of the very short head end and poste- 

 riorly into a well-developed tail (half-tail or tail bud). Hertwig 

 regards this interesting embryo merely as one in which the axial 

 half-structures developed on one side and not on the other. He 

 does not view or discuss it as illustrating a step in the direction 

 of forming a whole trunk out of one blastospore lip. And indeed 

 it may not illustrate such a step, although it obviously presents 

 a close analogy to the Chorophilus embryo described above. 



Endres and Walter ('95) described a frog embryo in which 

 possibly a process was going on similar to that in our asymmetri- 

 cal embryos. They, loc. cit., p. 41 and passim, find with Roux 

 that half-embryos, developing from one of the first two blasto- 

 meres, restore more or less completely through 'postgeneration' 

 the missing organs of the other side. In the case referred to 

 (Hemi-embryo sinister: F, loc. cit., pp. 43 to 48) there is a large, 

 dorsal, exposed yolk area. One blastopore lip is well organized 

 and passes backward from the head end into a well-developed 

 tail. The other, the 'post-generated' lip, is very imperfectly 

 organized. 



Ill 



The common or typical regulatory method of establishing the 

 type structure employed by embryos in which the closure of the 

 blastopore has been extensively interfered with, is generally 

 thought to be that in which both lips organize and come together 

 symmetrically in the midline (group 3, p. 172). And there seems 

 to be no doubt that this method is employed sometimes, although 

 observations on its occurrence in an actual embryo are scanty. 



