22 



SEX 



used to be believed that the simplest organisms 

 showed no sexual reproduction, but subse- 

 quent researches have shown that this is 

 very far from being the case. Many single- 



Fio. 3. The Bell -Animalcule Vorticella. A, a typical 

 individual showing contractile fibre (CF) inside a non- 

 contractile sheath, the twisted nucleus (N), a contractile 

 vacuole (CV), a food-vacuole (FV), the mouth, and the 

 surrounding cilia. B, an individual which has in part 

 divided into a cluster of small units or microzooids (Z). 

 C, an individual, which may almost be called female, 

 with which a minute individual or microzooid (M), which 

 may be almost called male, is conjugating. 



celled organisms unite in pairs and two become 

 one; a total conjugation in which there is 

 obviously so far a decrease, not an increase 

 in numbers. Others, such as many ciliated 

 Infusorians, show partial conjugation, that is, 



