VI.] THEIR SIGNIFICANCE IN HEREDITY. '^^'}^ 



moved before embryonic development can take place. In this 

 way it seemed to me that not only the ordinary cases of 

 ovogenetic and embryonic development became more easily 

 intelligible, but also the rarer cases in which one and the 

 same species produces two kinds of eggs — 'summer and 

 winter eggs.' Such eggs not only differ in size but also in the 

 structure of yolk and membranes, although identical animals 

 are developed from each of them. This result presupposes 

 that the nucleus in both eggs contains identical germ-plasm, 

 while the formation of different yolks and membranes requires 

 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 ovo- 

 genetic 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 h3^pothesis at least, we could 

 not conclude with certainty as to the necessity for a single 

 or a double division of the &g%. 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 the 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 ex- 

 planation of the physiological value of polar bodies ; leaving 

 the future to decide not only whether such explanation were 

 valid, but also whether it were exhaustive. The explanation 

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

 authorities. Hensen^ does not consider that my reasons for 

 the distinction between germ-plasm and histogenetic nucleo- 

 plasm are conclusive, and it may be conceded that this 

 objection was perhaps, at that time, well founded. O. Hertwig 



^ Hensen, ' Die Grundlagen der Vererbung,' Zeitschr. f. wiss. Land- 

 wirthschaft. Berlin, 1885, p. 749. 



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