THEIK SIGNIFICANCE IN HEREDITY. 355 



concerning- this process, but even if any one were ready to adopt it, 

 he would be unable to make any use of the idea. He would 

 not be able to support the theory in this way, for we now know 

 that nuclear substance is removed with the polar body, and this 

 fact requires an explanation which cannot be afforded by the 

 theory, if we are right in believing- that the expelled nuclear sub- 

 stance is not merely the indifferent bearer of the unknown principle 

 of the male condition, but hereditary substance. I therefore be- 

 lieve that Minot's, Balfour's, and van Beneden's hypothesis, al- 

 though an ingenious attempt which was quite justified at the time 

 when it originated, must be finally abandoned. 



My opinion of the significance of the second polar body is 

 shortly this, a reduction of the germ-plasm is brought about by its 

 formation, a reduction not only in quantity, but above all in the 

 complexity of its constitution. By means of the second nuclear 

 division the excessive accumulation of different kinds of hereditary 

 tendencies or germ-plasms is prevented, which without it would be 

 necessarily produced by fertilization. With the nucleus of the 

 second polar body as many different kinds of idioplasm are removed 

 from the egg as will be afterwards introduced by the sperm- 

 nucleus ; thus the second division of the egg-nucleus serves to 

 keep constant the number of different kinds of idioplasm, of which 

 the germ-plasm is composed during the course of generations. 



In order to make this intelligible a short explanation is necessary. 



From the splendid series of investigations on the process of 

 fertilization, commenced by Auerbach and Biitschli, and continued 

 by Hertwig, Fol, Strasburger, van Beneden, and many others, 

 and from the theoretical considerations brought forward by Pfliiger, 

 Nageli, and myself, at least one certain result follows, viz. that 

 there is an hereditary substance, a material bearer of hereditary 

 tendencies, and that this substance is contained in the nucleus 

 of the germ-cell, and in that part of it which forms the nuclear 

 thread, which at certain periods appears in the form of loops or 

 rods. We may further maintain that fertilization consists in the 

 fact that an equal number of loops from either parent are placed 

 side by side, and that the segmentation nucleus is composed in this 

 way. It is of no importance, as far as this question is concerned, 

 whether the loops of the two parents coalesce sooner or later, 

 or whether they remain separate. The only essential conclusion 



A a 2 



