398 Experimental Zoology 



followed much later by the discovery that a process similar to the 

 formation of the polar bodies takes place in the male germ-cells. 

 This seemed to be in contradiction to the hypothesis that the 

 elements of the one or of the other sex are ejected from the egg, 

 but the difficulty has not been found theoretically insuperable. 

 An examination of what takes place during the "maturation divi- 

 sions" of the egg and the sperm-cell will make this clearer. 



When the egg nucleus breaks down, preparatory to the forma- 

 tion of the first polar body (Fig. 25, 3), it is found that the chromo- 

 somes are only half as many as were present in the earlier or oogo- 

 nial divisions of the same germ-cells (Figs, i and 2). It is generally 

 admitted that this reduction in number is due to the chromosomes 

 having united in pairs. It is also supposed that, during this 

 pairing of the chromosomes, one derived from the male parent 

 (the paternal chromosome) unites with its homologous maternal 

 chromosome. In cases where the chromosomes are of very un- 

 equal sizes, there are, with rare exceptions, two of each kind in the 

 early germ-cells, one derived from each parent, and those of the 

 same size are supposed, as stated, to pair with each other (Fig. 3). 

 Therefore when the polar spindle is formed, the chromosomes, 

 half in number, really represent double chromosomes. These 

 chromosomes separate again when the first polar body is given 

 off, half going to one pole and half to the other pole of the spindle 

 (Fig. 4), so that some of the chromosomes in the first polar body 

 are maternal and the others paternal chromosomes. If all of 

 the paternal chromosomes were turned toward one pole of the 

 spindle, and all of the maternal toward the other pole, there would 

 be a complete separation of the chromosomes derived from the 

 mother from those derived from the father. But for certain 

 theoretical reasons it is supposed that this does not take place, 

 but that there is generally a haphazard separation leading to a 

 mixture of maternal and paternal chromosomes at each pole, 

 although one of each kind is present at each pole. 



The chromosomes that remain in the egg become quickly 

 arranged on a new spindle (Fig. 5). Each then splits length- 

 wise, as in ordinary cell divisions, and a half of each goes to one 



