22 J. T. PATTEKSON 



However, they never form a complete membrane, as Marehal 

 ('04) observed in the egg of Ageniaspis. 



We shall conclude this section by a statement concerning the 

 fate of the cells which inherit the nucleolar materials, as we 

 shall have no further occasion to refer in detail to that subject. 

 In text figure 2 I have outlined in diagram form the history of 

 the distribution of that body up to and including the twenty- 

 eight-celled stage. In certain instances the descendants of the 

 four nucleolar cells of the twenty-eight-celled stage may be 

 recognized (fig. 47, No. C), but beyond the seventy-celled stage 

 one can no longer follow their history, at least in preparations 

 made by the usual methods of technique. There is nothing in 

 the subsequent history of the egg to show that these cells have 

 been set aside for special function or that their behavior is 

 different from that of the descendants of the other three blasto- 

 meres. It is true Silvestri has formulated the very attractive 

 hypothesis that the nucleolar cells may become the primordial 

 germ cells for the sexual embryos which later develop. He has 

 apparently strengthened this hypothesis by his studies on the 

 development of the monembryonic egg of certain parasitic 

 species (Silvestri, '08) in which he was able to show that a similar 

 nucleolar-like body is included in the primordial germ cell, and 

 thus may be regarded as a germ-line determinant. 



Aside from the failure to trace these so-called germ cells to the 

 separate embryos, there are two other objections which are fatal 

 to his hypothesis. In the first place, it is impossible to conceive 

 of a mechanism which could operate in such a manner as to 

 parcel out exactly predestined germ cells to the several hundred 

 embryos. It would seem that some embryos might receive too 

 many germ cells, while others might receive none at all. To be 

 sure, his corollary hypothesis, that the asexual larvae owe their 

 asexuality to the absence of germ cells, would account for the 

 latter slip in the mechanism; but I have elsewhere ('17 a) pointed 

 out that these non-viable larvae are probably the result of an 

 entirely different cause. In the second place, I hope to show that 

 in some cases an embryo is derived from a single cell during the 

 late history of the polygerm. If this can be established as a 



