158 A. FRANKLIN SHULL 



the external conditions are properly selected; and any line with a 

 not too high proportion of male-producers can be made to yield 

 no male-producers. There is probably no such thing as a line 

 which, under all circumstances, yields no male-producers. But 

 even if we recognize that the internal factor is only one of several 

 which together determine the proportion of male-producers, the 

 internal agent may still not be zygotic constitution. It is quite 

 possible that something, perhaps environment, may permanently 

 modify the internal nature of a pure line, as Woltereck ('09) 

 believes may occur in the daphnians. Such a modified internal 

 nature could not be called zygotic constitution. Whether such a 

 modification can occur in Hydatina is unknown, but it is conceiv- 

 able; and so long as it is possible, we must not cling too tena- 

 ciously to zygotic constitution as the sole agent in producing the 

 internal differences described. 



It would be of great interest to know the process by which the 

 internal factor, whatever it is, affects the life cycle. Does the 

 cycle depend, internally, upon the quantity of something intro- 

 duced by the gametes, or does it depend on segregable genes that 

 combine and recombine in Mendelian fashion, or does it follow 

 some rule not analogous to anything known in other animals? 

 Furthermore, do any of these agents, external or internal, deter- 

 mine whether the cytological changes accompanying a change of 

 the mode of reproduction shall occur in a given cell, or do they 

 merely decide which of two already differentiated classes of cells 

 shall go on and develop? These questions must for the present 

 remain unanswered. There is no evidence on which to base an 

 answer to the second question ; while if we attempt to answer the 

 first, all we can say is that if the internal agents work either quan- 

 titatively or in a Mendelian manner, the process is a complicated 

 one. Results like those obtained from the crossing experiments 

 with Hydatina have not been reported, so far as I know, from any 

 other animal. A theory based on the few facts now known 

 regarding this rotifer would be highly speculative and might have 

 little value except as a pictorial representation to aid the memory. 

 I prefer, therefore, to obtain more data before attempting to 

 explain the results. 



