THEIK SIGNIFICANCE IN HEREDITY. 343 



supposes that the nucleus in both eggs contains identical germ- 

 plasm, while the formation of different yolks and membranes re- 

 quires the supposition that their nucleoplasm is different, inasmuch 

 as the two eggs differ greatly in histological character. 



The fact that equal quantities are separated during nuclear 

 division, led me to conclude further that the expulsion of ovogenetic 

 nucleoplasm can only take place when the germ-plasm in the 

 nucleus of the egg-cell has increased by growth up to a point at 

 which it can successfully oppose the ovogenetic nuclear substance. 

 But we do not know the proportion which must obtain between the 

 relative quantities of two different nuclear substances in order that 

 nuclear division may be induced ; and thus, by this hypothesis at 

 least, we could not conclude with certainty as to the necessity for 

 a single or a double division of the egg. It did not seem to be 

 altogether inconceivable that the ovogenetic nucleoplasm might be 

 larger in amount than the germ-plasm, and that it could only be 

 completely removed by means of two successive nuclear divisions. 

 I admit that this supposition caused me some uneasiness ; but since 

 nothing was known which could have enabled us to penetrate more 

 deeply into the problem, I was satisfied, for the time being, in 

 having found any explanation of the physiological value of polar 

 bodies ; leaving the future to decide not only whether such explana- 

 tion were valid, but also whether it were exhaustive. The explana- 

 tion seems to have found but little favour with some of our 

 highest authorities. Hensen l does not consider that my reasons for 

 the distinction between germ-plasm and histogenetic nucleoplasm 

 are conclusive, and it may be conceded that this objection was 

 perhaps, at that time, well founded. O. Hertwig does not mention 

 my hypothesis at all in his work on embryology 2 , 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 hypothesis is discussed by Hert- 

 wig, but Biitschli's 3 is preferred by him, although these two 



1 Hensen, ' Die Grundlagen der Vererbung,' Zeitschr. f. wiss. Landwirthschaft. 

 Berlin, 1885, p. 749. 



2 0. Hertwig, ' Lehrbuch der Entwicklungsgeschichte des Menschen und der Wir- 

 belthiere.' Jena, 1886. 



3 Butschli, ' Gedanken iiber die morphologische Bedeutung der sog. Richtungskor- 

 percheii,' Biol. Centralblatt, Bd. VI. p. 5. 1884. 



