230 



Sex-determination in Hydatina. 



observe the extrusion of a polar body, Lennsen* nevertheless describes the 

 beginnings of karyokinesis in the parthenogenetic ? eggs. The process, 

 however, appears to be arrested half way, and no segregation of chromatic 

 material seems to occur. Whether there is such segregation must be left for 

 future investigation to decide. The problem can be decided only by tracing 

 accurately the fate of the chromosomes, and it is with a view to stimulating 

 further work on the part of the histologist that I have put forward the 

 highly speculative suggestion outlined above. 



Before concluding these remarks on the nature of parthenogenesis a few 

 words may be devoted to another aspect of the problem. Since the publica- 

 tion of Maupas' experiments on ciliated infusorians it has been generally 

 held among biologists that the effect of conjugation is to arrest a process of 

 senile decay otherwise inevitable in animal stocks which exhibit the 

 phenomenon of asexual reproduction. In other words, there must be 

 a limit to the amount of protoplasm produced in the period between two 

 successive acts of conjugation. Among lowly organisms such as the 

 protozoa this amount may be very large. Among the more highly 

 differentiated metazoa it is relatively small, and where reproduction is 

 entirely sexual it is limited to the amount produced during the lifetime 

 of an individual. A pure thelytokous strain of Hydatina senta offers a 

 remarkable contrast to the rest of the metazoa. If we reckon the volume of 

 an individual at O01 cubic millimetre, and suppose that each is capable of 

 producing 30 eggs it is a matter of simple calculation to show that the 

 volume of protoplasm producible in 65 generations (i.e., in less than a year) 

 would form a solid sphere of which the dimensions would be such that 

 it could not be contained within the limits of the known universe.f We are, 

 therefore, driven to one of two alternatives — either that what may be termed 

 the potential productivity of protoplasm is immeasurably greater in Hydatina 

 than in all other metazoa, or that some process of conjugation occurs at one 

 stage or another in the so-called parthenogenetic reproduction of this form. 

 For reasons already stated I am inclined to favour the latter view, and it is 

 in the hope that the matter may attract the attention of the histologist 

 that I have ventured upon the calculation which forms the substance of this 

 last paragraph. 



* Loe. cit., p. 437. 



t For assistance in making the calculations upon which this statement is based I am 

 greatly indebted to my astronomical friend, Mr. Cookson, of Trinity College, Cambridge. 



