THE VITAL PROPERTIES OF THE CELL 277 



The theory built up by Weismann, Strasburger, Maupas, 

 Richard Hertwig, and myself may be worked out more in detail in 

 the following manner. Daring fertilisation two circumstances 

 must be considered, which work together and yet are opposed to 

 one another. In the first place, it is necessary for the nuclear 

 substances of the two cells to become mixed ; hence the cells must 

 be able to find one another and to unite. Secondly, fertilisation 

 affords the starting point for a new process of development and a 

 new cycle of cell divisions ; hence it is equally important that 

 there should be present, quite from the beginning, a sufficient 

 quantity of developmental substance, in order to avoid wasting 

 time in procuring it by means of the ordinary processes of nutri- 

 tion. 



In order to satisfy the first of these conditions, the cells must be 

 motile, and hence active ; in order to satisfy the second, they must 

 collect these substances, and hence increase in size, and this of 

 necessity interferes with their motility. Hence one of these 

 causes tends to render the cells motile and active, and the other to 

 make them non-motile and passive. Nature has solved the diffi- 

 culty by dividing these properties which cannot of necessity be 

 united in one body, since they are opposed to one another between 

 the two cells which are to join in the act of fertilisation, according to 

 the principle of division, of labour. She has made one cell active 

 and fertilising, that is to say male, and the other passive and 

 fertilisable, or female. The female cell or egg is told off to supply 

 the substances which are necessary for the nourishment and 

 increase of the cell protoplasm during the rapid course of the pro- 

 cesses of development. Hence, whilst developing in the ovary, it 

 has stored up yolk material, and in consequence has become large 

 and non- motile. Upon the male cell, on the other hand, the second 

 task has devolved, namely of effecting a union with the resting 

 egg. cell. Hence it has transformed itself into a contractile sperma- 

 tozoon, in order to be able to move freely, and, to as large an 

 extent as possible, has got rid of all substances, such as yolk 

 material or even protoplasm itself, which would tend to interfere 

 with this main purpose. In addition it has assumed a shape which 

 is most suitable for penetrating through the membrane which 

 protects the egg, and for boring its way through the yolk. 



We may transfer the terms male and female from the cell ele- 

 ments, which are thus differentiated sexually, to the nuclei 

 which they contain, even when these are equal both as regards 



