114 DEVELOPMENT OF THE FROG'S EGG [Cii. XI 



is due to the u yolk lying higher up on one side" is only begging 

 the question. Roux has not failed to notice the incomplete- 

 ness of Hertwig's explanation, and has interpreted all of Hert- 

 wig's results as due to a sudden postgeneration of the injured 

 half of the embryo; i.e. Roux believes a half -embryo to have 

 first formed, and then to have been quickly followed by an im- 

 perfect formation of the other half. Hence the asymmetry of 

 the embryos. 



It is impossible to sa}^ how far postgeneration has played 

 a part in the development of the embryos described by Hert- 

 wig, but that postgeneration will explain all the difference 

 between the results of Roux and of Hertwig seems highly 

 improbable. Further, as I have" said, it seems not unlikely 

 that many of the embryos described by Hertwig have come, 

 not only from the uninjured blastomere, but also from a part 

 of the injured blastomere. If this latter supposition be true, 

 we can better understand why the injured yolk forms in many 

 cases an integral part of the developing embryo. 



Hertwig has made a most formidable attack on Roux's expla- 

 nation of postgeneration of the embryo. The subject itself is of 

 secondary importance as compared with the main problem in- 

 volved in the experiment, and yet of sufficient interest to war- 

 rant careful examination. Roux describes the blastomere into 

 which the hot needle has been plunged as dead, and speaks of a 

 later revivification of the dead half of the egg. Hertwig be- 

 lieves that all of that part of the operated blastomere that is 

 later divided up into cells (to be used in the development) is 

 not dead, but only more or less injured. Only a small portion 

 of the injured blastomere is really dead, and that is the por- 

 tion which has become coagulated by the hot needle. This 

 portion cannot later be broken up into cells, but may be either 

 thrown out by the living embryo or assimilated, owing to the 

 power of digestion of neighboring cells. The injured blasto- 

 mere behaves in the same way that a portion of the body of an 

 animal would if a needle had been stuck into it. The place 

 injured might quickly heal, and the comparatively small region 

 that had been pierced and killed would be reabsorbed again. 

 If the needle had been first heated, the region of injury would 

 only be larger, and the necrotic tissue would be either thrown 



