354 ON THE NUMBER OF POLAR BODIES AND [VI. 



does not mention my h3'pothesis at all in his work on embrj'^o- 

 logy\ although he states in the preface: 'Among current 

 problems I have chiefly taken into consideration the views 

 which seem to me to be most completely justified, but I 

 have not left unmentioned the views which I cannot accept.' 

 Minot's hj'pothesis is discussed by Hertwig, but Butschli's'^ 

 is preferred by him, although these two hj'potheses are not 

 strictly opposed to each other ; for the former is a purely 

 physiological, the latter a purely morphological explanation. 

 I desire to lay especial stress upon the fact that my h3'pothesis 

 is simply a logical consequence from the conclusion that the 

 nuclear substance determines the nature of a cell. How this 

 takes place is quite another question, which need not be dis- 

 cussed here. If it is only certain that the nature of a cell is 

 thus determined, it follows that a cell with a certain degree 

 of histological specialization must contain a nucleoplasm cor- 

 responding to the specialization. But the mature egg also 

 contains germ-plasm, and there are only two possibilities by 

 which these facts can be explained : either the ovogenetic 

 nucleoplasm is capable of re-transformation into germ-plasm, 

 or it is incapable of such re-transformation. Now, quite apart 

 from the arguments which might be advanced in favour of one 

 of these two possibilities, the fact that a body is undoubtedly 

 expelled from the mature egg seems to me of importance, 

 while it is of even greater importance that this body contains 

 nucleoplasm from the germ-cell. 



It may be thought that the process, as supposed by me, is 

 without analogy, but such a conclusion is wrong, for during 

 every embrj'onic development there are numerous cell-divi- 

 sions in which unequal nucleoplasms are separated from one 

 another, and in all these cases we cannot imagine any way 

 in which the process can take place, except by supposing 

 that the two kinds of nucleoplasm were previously united in 

 the mother-cell, although their differentiation probably took 

 place only a short time before cell-division. Perhaps the new 

 facts which will be mentioned presently, and the views derived 



^ O. Hertwig, ' Lchrbuch dcr Entwicklungsgcschichte des Mcnschen 

 und der Wirbclthierc' Jena, 1886. 



■^ Butschli, ' Gedanken uber die morphologische Bedcutung der sog. 

 RichtungskOrperchen,' Biol. Ccntralblatt, Bd. VI. p. 5. 1884. 



