86 ACIPEKSER. 



By the continued extension of the epiblast the uncovered part of 

 the hypoblast has in the meantime become reduced to a small cir- 

 cular pore — the blastopore — and in surface views of the embryo has 

 the form represented in fig. 50 A, hl.p. The invagination 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 mesenteron 

 soon become divided into a pigmented hypoblastic epithelium adjoin- 

 ing the lumen of the mesenteron (fig. 51, en) and a mesoblastic 

 layer (Sgp), between the hypoblast and the epiblast. The mesoblast 

 is divided into two plates between which is placed the notochord^ (Ch). 



"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 appear- 

 ance. 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 

 medullary plate into the medullary canal. This, as shewn in fig. 51, 



Fig. 51. Transverse section through the anterior part of an Acipenser 



EMBRYO. (After Salensky.) 

 Rf. medullary groove; Mj). medullary plate; TF/;. segmental duct; C/t. notochord ; 

 En. hypoblast ; Sgj). mesoblastic somite ; Sp. parietal part of mesoblastic plate. 



is effected in the Tisual vertebrate fashion, by the establishment of a 



medullary groove which is then converted into a closed canal by the 



folding over of the sides. 



The uncovered patch of yolk in the blastoporic area soon becomes 



closed over ; and on the formation of the medullary canal the usual 



neurenteric canal becomes established. 



The further changes which take place are in the main similar to 

 / those in other Ichthyopsida, but in some ways the appearance of the 

 / embryo is, as may be gathered from fig. 52, rather strange. This is 

 ( mainly due to the fact that the embryo does not become folded off 

 / from the yolk in the manner usual in Vertebrates ; and as will be 

 ■' shewn in the sequel, the relation of the yolk to the embryo is unlike 



that in any other known Vertebrate. The appearance of the embryo 



^ Salensky believes that the notochord is derived from the mesoblast. I could not 

 satisfy myself on this i^oint. 



