196 PHYSIOLOGY CHAP. 



attach to them? The male cell or spermatozoon is perhaps the 

 smallest, the most mobile cell of the organism, and amongst the 

 most resistant to external agencies ; the ovum or female cell is 

 perhaps the largest, one of the least mobile, and is amongst the 

 most delicate and vulnerable. When it is expelled from the 

 Graafian follicle, the human ovum is a globule with a diameter of 

 200/x; the human spermatozoon is a very fine filament which 

 including the head measures 50 ^ in length. Evidently these 

 differences in size, form, mobility, and resistance are co-ordinated 

 with their different functions as sexual cells : the spermatozoon is 

 the more active element, destined to move in search of the ovum, 

 to fertilise it; the ovum is the less active element, destined to 

 receive the spermatozoon, to be fertilised by it, and to furnish 

 to the germ of the new being its first aliment. Nature, according 

 to the principle of division of labour, has allotted to the two 

 elements opposite sexual properties which would be irreconcilable 

 in one single element ; accumulating on the one hand, in the ovum, 

 the substances necessary for the nutrition and multiplication of 

 the cellular protoplasm necessary to keep pace with the rapid 

 development of the germ of the new being ; on the other hand, 

 reducing the spermatozoon to a contractile filament, divesting it of 

 the vitelline and protoplasmic substances which would impede its 

 motor activity, and giving to it a form capable of passing through 

 the protective covering with which the ovum is invested and 

 penetrating to the yoke. 



This differentiation of the two sexual cells becomes more and 

 more accentuated in the gradations of ascent of living beings, 

 probably by successive and continuous adaptation, or by hereditary 

 selection and transmission. It is to be noted, however, that it is 

 concerned chiefly with the secondary characters of the two 

 elements, and has no intimate relationship with the essential 

 phenomena of fertilisation, that is, with the copula interna of the 

 two sexual elements. This is in fact accomplished specially by the 

 union of the two nuclear portions of the male and female cells ; 

 each of these pronuclei contributes to the formation of the 

 complete nucleus of segmentation, which we have called " the germ 

 of the being to be born." Whatever the difference in size, form, 

 and properties between the ovum and spermatozoon, they con- 

 tribute to fertilisation nearly equal amounts of their active nuclear 

 substance, in which our means of investigation do not enable us 

 to discover the least difference. " There does not exist then 

 (wrote 0. Hertwig) a fecundating substance specifically feminine, 

 or a fecundating substance specifically masculine. The two nuclear 

 substances which unite in fecundation do not differ one from the 

 other, except in proceeding from two different individuals. Fecun- 

 dation is not the neutralisation of two sexual antitheses, because 

 these antitheses are only based on characters of a secondary order." 



