176 THE PHILOSOPHY OF BIOLOGY 



germ-cell, is fertilised or " activated " by the male 

 germ-cell. But this activation by the spermatozoon 

 is not necessary, for the ovum itself is capable of 

 division and development to form a complete organism. 

 This occurs in the cases of natural parthenogenesis 

 among insects and some other animals, where the 

 ovum proceeds, without fertilisation, to segmentation 

 and development. In some lower plants, where the 

 size of the male and female germ-cells is nearly equal, 

 either of them may undergo parthenogenetic develop- 

 ment : in such cases we cannot, of course, properly 

 speak of sexual differentiation. In the cases cf 

 organisms normally reproducing sexually, the stimulus 

 to development is afforded by the entrance into the 

 ovum of the spermatozoon — that is, by the mixture 

 of the male and female germ-plasms ; but in some 

 animals this stimulus may be replaced by the addition 

 to the water in which they are living of certain chemical 

 substances. This is the process of artificial partheno- 

 genesis first studied by Loeb in the case of the eggs 

 of the Sea-urchin ; and its anatysis suggests that the 

 spermatozoon conveys some substance into the egg, 

 and that this substance initiates segmentation by 

 setting up a train of chemical reactions. What these 

 reactions are exactly, and what is the process of 

 " formative stimulation " by the spermatozoon, we 

 do not know. It is quite certain, however, that much 

 more than this process of formative stimulation is 

 involved in the fertilisation of the egg by the sperma- 

 tozoon. The mixture of the male and female germ- 

 plasms resulting from conjugation confers upon the 

 embryo the characters of both the parents and of 

 their ancestries. 



In an unicellular organism the " body " consists 

 of a single cell containing a nucleus. The extra- 



