100 DEVELOPMENT OF llETERODOXTUS (CESTRACION) PHILLIP I, 



of the parablast of its I'oof appears to divide into cells which 

 are added to the posterior part of the blastoderm. A distinct 

 upper layer of the blastoderm (" ectoderm" of various authors) is 

 no longer recognisable. 



C. K. Hoffmann* states that in Acanthias, at a stage in the 

 development of the blastoderm which corresponds broadly with that 

 just described, there is an invagination- or gastrula-cavity opening 

 widely by a blastopore on the exterior. The mode of formation 

 of this gastrula-cavity, he avers, is closely comparable to the 

 gastrulation in Amphil)ia, Cyclostomi and Amphioxus. If we are 

 to accept the statement that the cavit}^ in question is a gastrula 

 cavity, then necessarily^ we must admit the justness of the com- 

 parison with the corresponding cavities in other Chordates. 

 Such an admission, however, would involve us in the greatest 

 difficulties. For here we should have an invagination Avhich is 

 not connected with the formation of the archenteron or of the 

 mesoderm or notochord, an invagination-cavity which virtually 

 disappears before the first rudiment of the mesoderm has become 

 differentiated. I do not think, however, that the statement of 

 fact can be taken without confirmation, and am confident that 

 more thorough investigation will show that Acanthias does not 

 depart so widely from other Elasmobranchs in such an essential 

 phase of its development. I have several series of sections of 

 blastoderms of Heterodontus at or about the stage represented in 

 fio-. 3. These, so far as they were examined in the fresh state, all 

 presented the appearance described by Hoffmann, an appearance 

 seeming to indicate the presence of an open cavity below the^ 

 posterior edge of the blastoderm. In one of them only does the 

 cavity open on the exterior; and in this the opening is readily seen 

 on a careful examination to have resulted from a rupture of the 

 delicate roof of the cavity, most j)robably during the removal of 

 the blastoderm from the egg. 



« " Beitiiige zur Entwickehingsgeschichte der Selachii," Morph. J.B. 

 1896. 



