The Embryology of Chlamydoselachus 



583 



down with little change from Palaeozoic times". It is interesting to note the occurrence 

 of this spot on the egg of that shark (Chlamydoselachus) to which systematists have as- 

 signed the lowest rank among recent elasmobranchs. 



Nishikawa (1898) also had older blastoderms of Chlamydoselachus. He says ''The 

 next stage was a blastula, with a distinct segmentation cavity, whose floor was bounded 

 by what has been termed 'periblast' with fine granular yolk, and merocytes with vacuo- 

 lated protoplasm, due perhaps to the dissolution of the contained oil drops, and many 

 nuclei. One end of the blastula was thicker than the other, and is evidently the 'em- 

 bryonic end' of Balfour, and the 'anterior end' of Riickert". Unfortunately Nishikawa 

 does not figure the blastoderm on the yolk, nor the entire blastoderm either in surface or 

 sectional view, nor does he give the size to which it has grown. It is greatly to be re- 

 gretted that Nishikawa did so little with this precious early material. 



CU,t.o i ijij} (, 



Text-figure 22 

 Diagrammatic sketches representing the 

 cleavage pattern in four different types of 

 vertebrate eggs: A, probably a hypo- 

 thetical type ancestral to elasmobranchs; 

 B, Chlamydoselachus; C, sharks; and D, 

 a type reverting from the meroblastic to 

 the holoblastic condition. 

 Sketches by Bashford Dean. 



In Dean's notebook labelled Chlamtdoselachus there are in various places 

 notes on eggs and embryos obtained during his two visits to Japan, or collected after 

 each visit and sent to him in America. One paragraph is labelled "Material and 

 List of Figures". Here I find "? Blastula", and on another page "Apr 10, 3 blastulae". 

 He makes no specific mention of early blastulae — i.e., of early segmentation stages. 

 Whether the alleged "blastulae" were in early or late stages is not known since Dean had 

 no surface drawings made and since no preserved specimens can be found among his 

 materials. If he had live eggs of Chlamydoselachus, perhaps he had the same trouble in 

 finding early blastoderms that I had with live eggs of Ginglymostoma — i.e., that they 

 were so small that he overlooked them, since these yellow spots would be obscured by the 

 brownish-yellow capsules, as they were in Ginglymostoma by its thicker and darker 

 capsules. This difficulty would be increased in preserved eggs since the "white" (a thin 

 layer of glairy fluid) would be coagulated and some of the color of the germinal area would 

 be destroyed by the preservative. 



