Reminiscence of Holoblastic Cleavage in the Egg 

 of the Shark, Heterodontus (Cestracion) 



japonicus Macleay. 



BY 



Bashford Dean. 



In studies on the yolk characters of vertebrates the eggs of Elas- 

 mobranohs have always been described as presenting typical meroblastic 

 cleavage. This character has here indeed been deemed so axiom-like 

 that we find that the meroblastic egg is usually, if not always, made the 

 point of departure in various works dealing with the problem of the gain 

 or loss of yolk in eggs of the gnathostomes. Thus it has long been 

 discussed whether the eggs of the higher classes have been capable of 

 losing and reacquiring food yolk, or whether the eggs of the highest forms 

 have merely lost the ancestral supply. This, however, illustrates but 

 one line of discussion in which the embryologist has treated the eggs of 

 elasmobrauchs as irrevocably meroblastic. It is upon strictly theoretical 

 grounds (reducing the matter to its simplest terms, for as a matter 

 of fact segmentation has been observed in but very few forms) that 

 the early characters of the eggs of elasmobrauchs have been compared 

 with those of other ichtbyopsids. But in spite of this no one, I fancy, 

 would have been bold enough to have prophesied that the wide difference 

 between the typically meroblastic egg of the shark and the holoblastic 

 egg of such a teleostome as a sturgeon might come to be bridged over 

 within the limits, not of fossil sharks, but of recent sharks themselves. 

 Indeed it seemed all that embryoogists could hope for, that meroblastic 

 features should be discovered in such ancient forms of teleostomes 

 as Lepidosteus and Amia. It was naturally, therefore, a great surprise 

 to me to find in my studies on the development of Cestracion that there 

 occurred in this ancient shark features which are best interpreted as 



