24 DEAN. [Vol. XI. 



epidermic layer {cp), ' Deckschicht,' now appears to aid in the 

 process of closure ; at the margin of the blastopore it thickens 

 and appears especially distinct, and protrudes over the rim of 

 the blastopore, as it does in fact in Teleost : its innermost 

 cells, however, are still connected, but loosely, with those of 

 the inner layer : the continued reduction of the blastopore now 

 results in a funnel, closed at its pointed end by the inwedging 

 epidermic cells ; closure and fusion of the lips of the blasto- 

 pore next follow by what appears a concrescence of the sides 

 of the funnel, an obliteration completed as the caudal eminence 

 of the embryo is in process of forming. It will accordingly be 

 seen that the observation of Beard as to the absence of a neu- 

 renteric canal is thus confirmed. 



The view of that author, however, as to the origin of the 

 middle layer "from the epiblast on each side of the middle line, 

 and from the epiblastic region at the lip of the blastopore " has 

 not been verified. It will be seen in the above section that the 

 mesoblast (/;/) is clearly to be traced to its union with the hypo- 

 blast (or more accurately, perhaps, the undifferentiated tissue) 

 in the underlying region of the blastopore. Its growth thence 

 extends on all sides, but most rapidly forward in the direc- 

 tion of the embryo's axis : in the transverse section, PI. II, 

 Fig- 33> gastral mesoderm is present ; its connection with 

 the inner layer was observed only in the hinder portion of the 

 embryo. 



In a sagittal section of an embryo of about 60 hours, PI. I, 

 Fig. 18, the region of the origin of the vascular system is of 

 considerable interest. The outer layer, after its thickening at 

 the head end of the embryo is seen suddenly to taper away as 

 it slants downward continuously over the depressed area ; it 

 now consists of the epidermic stratum, which has already 

 been noted, and an irregular single-celled layer of formative 

 epiblast, and in this condition it is continued around the yolk, 

 PI. II, Fig. 34 (in this figure the drawing has not been accurately 

 reproduced). The segmentation cavity was last seen in PI. II, 

 Fig. 31 ; its flattened condition in that stage has now given rise 

 to the vascular disc-shaped enlargement immediately in front of 

 the embryo's cephalic eminence ; in other regions it has become 



