The Embryology of Heterodontus japonicus 



737 



Text-figure 55. 



Transverse section through the posterior portion 



of a blastoderm of Heterodontus phiHipt in a stage 



somewhat later than the preceding, but before 



the differentiation of the notochord. 



ect, ectoderm; end, entoderm; ent, archenteric 

 After Haswell, 1898, Fig. 8, pi. V. 



cavity. 



end 



&CL 



In Heterodontus phillipi, as in other Elasmobranchs, gastrulation takes place mainly 

 by a process of involution. According to Haswell (1898) the first phase of gastrulation 

 consists of an arching upward of the posterior portion of the blastoderm, so that where it 

 passes into the parablast it becomes, for a short distance, vertical. It soon inclines forward 

 (as illustrated in my Text'figure 53), forming the embryonic rim which extends along the 

 entire posterior margin of the blastoderm. At the same time the accumulation of cells at 

 the posterior end increases at the expense of the segmentation cavity. In the stage 

 represented by Text'figure 53 the segmentation cavity has become extremely shallow, and 

 its roof has acquired a compact epithelial character. 



The stage represented by Text'figure 54 possesses a definite entoderm, hence it is 

 a well'established gastrula. To the present writer it seems that the cleft, separating the 

 poorly'defined layer of cells marked end' from the irregular layer above, is mainly an 

 artifact. Text'figure 55 is a transverse section through the archenteric cavity of a stage 

 a little farther advanced than the one shown in Text'figure 54. It shows no new features, 

 but is helpful in afibrding a different point of view. At first the floor of the archenteric 

 cavity consists only of yolk; but soon the anterior portion of the archenteron will form an 

 entodermal pocket, the fore'gut. 



Text-figure 56. 



Sagittal section, approximately median, of Heterodontus (presumably japonicus) in an advanced gastrula stage, 

 ant, anterior; ect, ectoderm; end, endoderm; fier, periblast; per', periblast endoderm; vac, vacuole. 

 Drawn from a slide simply labelled Cestracion, in the collection left by Bashford Dean. 



