No. 2.] FORMATION OF THE FISH EMBRYO. 46 1 



artificial division of it into two parts, Urmundrancl and Keim- 

 scheibenrand, one equivalent to the gastrula mouth, the other 

 not (the process "a modified gastrulation process"), seems to 

 be an unwarranted assumption. The differences in the germ- 

 ring in different regions are quantitative, not qualitative. 

 Sooner or later the whole of the material of the under layer 

 (and probably a part of the upper layer) of the germ-ring passes 

 into the embryo. 



The supposed insuperable difficulty of accepting this fact, 

 which is the basis of His' theory, lies in the value which His 

 has given to the ring itself ; for in it he sees the two sides of 

 the embryo. If, on the other hand, we accept the fact that 

 the germ-ring slowly passes into the embryo but forms only a 

 comparatively small portion of it, then the criticisms of Oellacher 

 and Hertwig fall to the ground. We return thus to the dis- 

 covery of Lereboullet and His, that the germ-ring does go into 

 the embryo. To this we may add the modern conception, 

 which does not originate with Hertwig, however, that the germ- 

 ring of the fish is the blastopore. Finally, the results of 

 experimental studies show us that the median portion of the 

 fish may elongate posteriorly independently of the additions 

 from the germ-ring ; and from this median portion, early laid 

 down on the blastoderm, develops the greater part of the body 

 of the fish. 



