460 MORGAN. [Vol. X. 



I may, say at once that I do not believe his view of the sub- 

 ject to have been a happy choice, and, further, I think Hertwig 

 has failed to give sufficient weight to some of the most im- 

 portant facts of teleostean development. Hertwig says : " In 

 my conception of the gastrulation processes in the teleost egg 

 I agree with His to this extent, that we both look upon the 

 back of the embryo as formed by the union of lateral halves. 

 In other respects I differ from him on very important points. 

 According to my view, the whole appears as a modified gas- 

 trulation process. The parts that unite are the lips of the 

 gastrula mouth, and the periphery of the germinal disc (Keim- 

 scheibenrand) is gastrula mouth only so far as it takes part in 

 the fusion process [apposition]. , , According to His the 

 structures that lie later in the median plane of the body form 

 at first the whole periphery of the germ-disc. The whole 

 border zone of the disc (Randwulst der Keimscheibe) is used 

 up to form the embryo, and it is used up at the same rate that 

 the overgrowth takes place, so that the embryo is completed 

 at the same moment the yolk-sphere is covered," " Oellacher 

 has brought forward this objection to Lereboullet's theory 

 (which is very similar to that of His): If the rim of the germ- 

 disc forms the two halves of the body, then must these halves 

 run around the whole ^g^ and at one time form a great 

 circle of the sphere. Therefore, the two halves of the body 

 must be at one time enormously drawn out, which is certainly 

 not very probable." Hertwig adds : " This difficulty is absent 

 from my interpretation, and I can justly say the concrescence 

 theory of His appears in my < Urmundtheorie,' but in a form 

 with modifications that are not unimportant." 



It seems to me that Hertwig has completely misinterpreted 

 the facts of teleostean development; that his "not unimportant" 

 modification of His' theory lies entirely in a wrong direction. 

 While I agree with Oellacher and Hertwig in their criticism of 

 His' view of the germ-ring as actually representing the two 

 halves of the embryo, I also think that Hertwig has gone much 

 farther from the actual facts than even His has done. 



The homogeneity of the germ-ring will be apparent to any 

 one who has studied the development of a fish, and Hertwig's 



