i INTEODUCTION 13 



with the function of producing the peculiarities of the cytoplasm 

 of the unripe egg the amount, colour, and composition of the food- 

 yolk, etc. When this task was accomplished and the ovum was ripe, 

 it was supposed that this portion of the nucleus, termed by Weis- 

 mann histogenetic plasm, was extruded as the first polar body. 

 The extrusion of the second polar body he explained by his theory 

 of "ids." 



According to this theory, the material basis of the transmission of 

 the parental qualities to the child is contained in the chromatiu 

 which is organized into a number of " ids." Each " id " contains 

 within itself the whole potentiality of the animal, i.e. one "id" alone 

 is capable of causing the egg to develop into an adult animal. The 

 " ids " are similar, but not exactly the same, and the animal which 

 develops is a compromise between the potencies of the various " ids." 

 The " ids " are capable of assimilation and growth, and in each 

 longitudinal division of the chromosome each " id " contained therein 

 is divided into two precisely similar daughter "ids," but in the 

 transverse or reducing division (which in Weismann's day was 

 supposed to be the second, not the first division) different " ids " are 

 separated from one another. 



This is what happens in the formation of the second polar body, 

 and now the spermatozoon brings in an equal number of " ids " and 

 thus the number originally present in the oogonia is restored. Since 

 the " ids " of the spermatozoon are not exactly the same as those of 

 the egg, and since Weismann assumed that the casting forth of half 

 the maternal " ids " might take place at random, i.e. might consist of 

 any group of the "ids " amounting to half the number, the basis was 

 given for inheritable variations, because different combinations of 

 maternal and paternal " ids " might come about by a difference in 

 the " ids " which were cast forth at the reducing division. 



It is one great merit of Weismann's theories that, whatever may 

 be thought of their truth or untruth, they have acted as powerful 

 stimulants to research. Hertwig (1890) published a work on the 

 spermatogenesis and oogenesis of the worm, Ascaris megalocephala, 

 in which he showed, for the first time, the parallelism in the changes 

 which occur during the ripening of both kinds of germ cell, and 

 proved that the polar bodies were the degenerate sisters of the egg, 

 thereby overthrowing Weismann's theory of the histogenetic plasm. 

 But Weismann's theory of the meaning of the reducing division 

 was not thus disproved, although of course it was shown that it was 

 the first, not the second maturation division in which whole chromo- 

 somes were separated from one another, and to which Weismann's 

 hypotheses must apply. This theory was shattered by the work 

 which ensued on the discovery of what have been called sex- 

 chromosomes, by McClung, Wilson, and other American workers. 

 Wilson has given an excellent summary of the whole subject (1911), 

 and to this we must now address ourselves. 



In the spermatids of certain insects it was observed that in some 



