ox THE OASTRULATIOX TN PETROMYZON. 15 



to the correspondinp; structure, for instance, in Amphibian ova. 

 A narrow but sharply defined groove {M.g. 10 /;, r.r.) running 

 lengthwise along the median line of this oval field represents the 

 " Rückenrinne." It is broader and deeper towards the blastopore 

 and communicates with it {Figs. 10 h and 10 c, r.r.), while an- 

 teriorly it fades away. 



The gastrulation in the present condition is not yet closed, 

 but has yet to envelope the opaque hemisphere which is still 

 exposed between the blastopore and the boundary groove that has 

 just disappeared. This exposed part is brought within to some ex- 

 tent, doubtless by the active invagination of the opaque field itself, 

 which has been uninterruptedly taking place from the first : it is 

 flattened and dej^ressed in the like manner as in the preceding 

 stages. An active growth of the dorsal blastoporic lip is, however, 

 indisputable from the fact that the medullary plate is obviously 

 added by stages in its antero-posterior length. This addition in 

 the plate is indeed not less than one-third of the original length, 

 by the time a prominent pair of the medullary ridges becomes 

 visible (compare Figs. 10a and 10b with Figs. 11« and lib). 

 Such an enormous growth of the blastoporic lip denotes nothing 

 else than its overgrowth on the opaque field, which accomplishes, 

 under co-operation of the invagination of the opaque field itself, 

 the process of gastrulation. Now the blastopore assumes a circular 

 outline ; the precise mode by which the pore is brought into the 

 definitive form has already been stated in foregoing pages. 



I will now turn to follow the internal changes corresponding 

 to the superficial occurrences described in the preceding pages. 

 Let us start again from a young blastula stage. Fig. 12 shows 



