232 LECTURES ON BIOLOGY 



As the ' wander-germs ' which separate from the mother- stem 

 must often seek their partner in a comparatively enormous space 

 it is clear that numerous individuals must perish before they 

 reach the desired goal. It was therefore necessary to produce a 

 very large number of free-moving germs. The best way to pro- 

 duce such large numbers was at the expense of the body. Here 

 we have a natural explanation of the fact that the free-moving 

 individuals were developed as microgametes, or, as we may 

 also call them, male germs, while the others, the macrogametes, 

 or females, which remained fixed to the stem and had to await 

 the coming of the microgamete, could retain their original cell- 

 size. And as the small quantity of protoplasm made it probably 

 difficult for the microgametes to reproduce from themselves 

 alone normal individuals the permanent complete fusion with 

 a inacrogamete seemed most advantageous for their further 

 development. 



If this assumption sounds at first somewhat forced it finds, 

 nevertheless, corroboration in many facts derived from the life- 

 history of the higher animals. It is known that in these the 

 difference in size between male and female germ-cells has become 

 still more considerable than in the unicellular organisms, for the 

 ova belong to the largest, the spermatozoa to the smallest, 

 animal-cells known to us. The ovum of the sea-urchin exceeds 

 in volume the male germ by more than 200,000 times, and in 

 animals with unusually large eggs, in particular reptiles and 

 birds, the proportion becomes still more unfavourable to the 

 spermatozoon. The simple reason is that ovum-cells are pro- 

 duced in much less numbers than the spermatozoa, consequently 

 they are able to develop their protoplasm very strongly and load 

 themselves, in addition, with abundant quantities of foodstuff 

 for the maintenance of the growing embryo during the later 

 stages of its development. 



Spermatozoa on the other hand, being minute, are produced 

 in enormous quantities. Because it is necessary that they 

 possess the utmost motility to seek and find the ovum we 

 observe how during growth the spermatozoa reject almost their 

 entire protoplasm, so that a fully developed spermatozoon 

 consists only of nuclear substance. 



Although the spermatozoon contains, like the ovum, in its 



