174 THE PHILOSOPHY OF BIOLOGY 



their nuclei fuse, or rather some of the material of one 

 nucleus is transferred to the other. The cells then 

 separate and reproduction by division begins again. 



This is not necessarily sexual reproduction : it is 

 the conjugation of essentially similar morphological 

 entities. If two conjugating Paramcecia possessed 

 distinct personalities we might imagine a merging or 

 addition of two conscious durations or memories. 

 Sexuality, however, includes less than this. In this 

 mode of reproduction the conjugating bodies are not 

 organisms in the usual sense, but rather modified 

 organisms or highly modified parts of organisms. In 

 some lower plants the conjugating cells may be modi- 

 fied with respect to the cells characteristic of the 

 organism, but they may be approximately equal in 

 size. But in the multicellular plant and animal, in 

 general, the conjugates are cells detached from the 

 parental body, and differing chiefly from the cells of 

 the latter in that they show a lack of differentiation. 

 One of these cells, that detached from the paternal 

 body, is the spermatozoon (in the case of the animal), 

 or the pollen cell (in the case of the plant). It is 

 much smaller than the sexual cell detached from the 

 maternal body : this is the ovum in the case of the 

 animal, or the oosphere in the case of the plant. In 

 general the ovum is a relatively large cell, since it 

 contains abundant cytoplasm, which may also be 

 loaded with yolk or other reserve food material. 

 The spermatozoon is very much smaller and consists 

 of a nucleus with a minimal mass of cytoplasm. The 

 ovum is, in general, immobile ; the spermatozoon is 

 generally highly mobile. 



The essential process in the sexual reproduction 

 of the unicellular organisms is therefore the con- 

 jugation of the organisms themselves. In multi- 



