104 ACIPENSER. 



frog or the lamprey, and I must refer the reader for the details 

 of the process to the chapter on the Amphibia. The edge of the 

 cap of epiblast forms an equatorial line. For the greater extent 

 of this line the epiblast cells grow over the hypoblast, as in an 

 epibolic gastrula, but for a small arc they are inflected. At the 

 inflected edge an invagination of cells takes place, underneath 

 the epiblast, towards the segmentation cavity, and gives rise to 

 the dorsal wall of the mesenteron and the main part of the 

 dorsal mesoblast. The slit below the invaginated layer gradually 

 dilates to form the alimentary cavity ; the ventral wall of which 

 is at first formed of yolk-cells. The epiblast along the line of the 

 invaginated cells soon becomes thickened, and forms a medullary 

 plate, which is not very distinct in surface views. The cephalic 

 extremity of this plate, which is furthest removed from the edge, 

 dilates, and the medullary plate then assumes a spatula form 

 (fig. 50 A, Mg\ 



By the continued extension of the epiblast the uncovered 

 part of the hypoblast has in the meantime become reduced to a 

 small circular pore the blastopore and in surface views of the 

 embryo has the form represented in fig. 50 A, bl.p. The invagi- 

 nation of the mesenteron has in the meantime extended very far 

 forwards, and the segmentation cavity has become obliterated. 

 The lip of the blastopore has moreover become inflected for its 

 whole circumference. 



The invaginated cells forming the dorsal wall of the mesen- 

 teron soon become divided into a pigmented hypoblastic epithe- 

 lium adjoining the lumen of the mesenteron (fig. 51, En) and a 

 mesoblastic layer (Sgp], between the hypoblast and the epiblast. 

 The mesoblastis divided into two plates, between which is placed 

 the notochord 1 (Cli). 



With the completion of the medullary plate and the germinal 

 layers, the first embryonic period may be considered to come to a 

 close. The second period ends with the hatching of the embryo. 

 During it the rudiments of the greater number of organs make 

 their appearance. The general form of the embryo during this 

 period is shewn in figs. 50 B and 52 A and B. 



One of the first changes to take place is the conversion of the 



1 Salensky believes that the notochord is derived from the mesoblast. I could 

 not satisfy myself on this point. 



