THEORIES OF POLYEMBRYONIC DEVELOPMENT 91 



anlages, be they two, or four, eight, or eleven (a possible 

 number in Talusia hybrida^), is the lineal descendant 

 of a single blastomere, formed during early cleavage. 



THE FISSION THEORY 



In a paper read before the Ninth Zoological Congress 

 at Monaco, Assheton criticizes both the ''blastotomy'' 

 and the "budding" theories of NewTiian and Patterson. 

 The theory of budding seems to him especially unaccept- 

 able. "One cannot have budding," he says, "unless 

 there is a stock from which budding takes place. There 

 is nothing in Tatusia [Dasypus] one can call a stock. 

 The phenomenon is clearly that of fission." 



In support of the fission hypothesis, he cites evidence 

 derived from the embryological study of other mammals, 

 notably the sheep and the ferret. "In the sheep 

 [Ovis] I found some years ago a blastocyst at the stage 

 just before the formation of the embryonal areas with 

 two distinct ectodermic masses lying within the tropho- 

 blast." His outline figures show this interesting sheep 

 blastocyst (Figs. 35 and 36) to be covered by a complete 

 envelope of trophoblast and lined internally with a 

 complete layer of endoderm. Such a condition could 

 not have resulted from budding. 



In the ferret {Putorius) certain interesting conditions 

 (Figs. 37 and t^^) were found that seemed to show evi- 

 dences of a separation of blastomeres, but in no case 

 were twin embryos produced. These cases are cited 

 to prove "that fission of the embryonic rudiments of 

 eutherian mammals may be effected fairly easily, but 

 the occurrence is the exception, not the rule." A 



^ Meaning Dasypus hyhridus. 



