REPRODUCTION. 669 



reproductive cell ; when the reduction does not take place 

 the product is not a gamete, but a parthenogenetic spore. 

 We accept Balfour's view, that the extrusion of the polar 

 body is the means by which parthenogenesis is prevented, 

 and, we may add, the means by which cross-fertilisation, 

 with its attendant advantages, is rendered possible. A 

 gamete is, as a rule, converted into a spore by means of 

 the sexual process ; two reproductive cells, neither of which 

 contains sufficient nucleo-idioplasm for independent germi- 

 nation, form by their coalescence one which does. But it 

 appears that in some of the lower forms, the necessary 

 increase of the nucleo-idioplasm may be effected by nu- 

 trition, so that a gamete may become converted into a 

 spore without the sexual process. 



Having arrived at these conclusions respecting the sexuality 

 of gametes, we go on to enquire into the nature of sex. 



We have learned that Strasburger and Weismann are of 

 opinion that male and female gametes, oospheres and an- 

 therozoids for example, or at least their nuclei, are essentially 

 similar. From this point of view it is then merely the 

 external adaptive peculiarities of the gametes which con- 

 stitute their sex. But we shall endeavour to shew that this 

 opinion is not in harmony with the known facts of repro- 

 duction. 



In the first place it appears that, as a matter of fact, there 

 is a material difference between male and female gametes, at 

 least when they are highly differentiated. Zacharias has 

 found, as the result of extended observations on the an- 

 therozoids and oospheres of Characeae, Muscineae, and Ferns, 

 and on the oospheres and pollen-tubes of Phanerogams, that 

 the nucleus of the male cell has either no nucleolus or but a 

 small one, whereas that of the female cell has one or more 

 large nucleoli ; and further, that the male nucleus is rich in 

 nuclei'n, whereas the female nucleus is poor in nuclei'n, but 

 rich in albuminous substance. Much stress will not, however, 

 be laid upon these facts, for, after all, the material difference 

 between a male and a female gamete upon which the physiolo- 

 gical difference between them essentially depends, may be 



