356 ON THE NUMBER OF POLAR BODIES AND [VI. 



difference between an egg which is capable of developing 

 without fertilization, and another which requires fertilization, 

 must lie in the quantity of nucleoplasm present in the egg. I 

 supposed that the nucleus of the mature parthenogenetic egg 

 contained nearly twice as much germ-plasm as that contained 

 in the sexual egg, just before the occurrence of fertilization ; 

 or, more correctly, I believed that the quantity of nucleoplasm 

 which remains in the egg, after the expulsion of the polar 

 bodies, is the same in both eggs, but that the parthenogenetic 

 egg possesses the power of doubling this quantity by growth, 

 and thus produces from within itself the same quantity of 

 germ-plasm as that contained in the sexual egg after the ad- 

 dition of the sperm-nucleus in fertilization. 



This was only an h3'pothesis, and the considerations which 

 had led to it depended, as far as they went into Retails, upon 

 assumptions ; but the fundamental view that the quantity of the 

 nucleus decides whether embryonic development takes place 

 with or without fertilization seemed to me, even at that time, to 

 be correct, and to be a conclusion required by the facts of the 

 case. Indeed, I thought it not unlikely that its validity might 

 be proved by direct means : I pointed out that a comparison of 

 the quantities of the nuclei in parthenogenetic and sexual eggs, 

 if possible in the same species, would enable us to decide the 

 question {I.e., p. 239). 



I had thus set myself the task of making this comparison. 

 The result of this investigation was to show that, as already 

 mentioned, polar bodies are formed in parthenogenetic eggs. 

 But even the first species successfully investigated revealed 

 a further fact, which, if proved to be wide-spread and cha- 

 racteristic of all parthenogenetic eggs, was certain to be of 

 extreme importance : — the maturation of the parthenogenetic 

 egg is accompanied by the expulsion of one polar body, or, as 

 we might express it in another way, the substance of the 

 female pronucleus is only once divided, and not twice, as in the 

 sexual eggs of so many other animals. If this difi'crence 

 between parthenogenetic and sexual eggs was shown to be 

 general, then the foundations of my hypothesis would indeed 

 have been proved to be sound. The quantity of nuclear sub- 

 stance decides whether the egg is capable of undergoing em- 

 bryonic development. This quantit}^ is twice as large in the 



