No. 2.] FORMATION OF THE FISH EMBRYO. 453 



Ruckert conceives the whole periphery of the germ-disc to repre- 

 sent the blastopore, but only one portion of this to pass into the 

 embryo. This takes place by a process of concrescence which 

 serves to form the posterior portion of the embryo. If I 

 mistake not, this is the same view which Hertwig, in 1892, 

 advanced as his conception of the process of concrescence ; 

 but this paper of Riickert's is not quoted in Hertwig' s extensive 



bibliography (3). , ^ c ^. ^ 



In my preliminary paper on the development of the hsh, i 

 described the results of the experiments of cutting the germ- 

 ring on one side of the embryo. At this time I was not aware 

 that the same experiment had been successfully done by Kast- 

 schenko (7) in Selachians. To him, therefore, belongs the 

 credit of having first shown that under these conditions the 

 embryo continues to elongate posteriorly. 



Kastschenko cut the germ-ring to one side of the embryo 

 (stage VII). Nevertheless, a normal embryo developed that 

 lived through the next stage (VIII). In another experiment 

 the posterior end of the blastoderm was destroyed. The 

 anterior half of the embryo developed normally, but the pos- 

 terior half did not develop. The development of the posterior 

 half ought still to have followed, if His' hypothesis were true, 

 because the material for the latter was left uninjured in the 

 germ-ring. 



In a third experiment of Kastschenko' s the entire embryo (at 

 stage VII) was separated at the two sides, from its connection 

 with the rest of the disc, and both halves of the body continued 

 to develop up to the time of appearance of three protover- 

 tebrae. In some cases the growth of the two halves of the 

 embryo continued till the tail-folds appeared. 



The results of these experiments, Kastschenko says, con- 

 vinces him that His' theory is not true, and that the material 

 for the formation of the axial" portion of the embryo's body is 

 laid down from the beginning, not in the germ-ring, but in the 

 hinder end of the germ-disc. 



It will be seen that my own experiments of a similar nature 

 on the Teleost have led me to a like conclusion. In my 

 earlier paper I inferred, as does Kastschenko, that the germ- 



