THE HORMONES IN HUMAN REPRODUCTION 



activity and becomes the parent of a new clone. This process 

 is very much like fertilization of an egg by a sperm cell in 

 higher animals, and we can get from it a better understanding 

 of the meaning of sexual union. For example, the late Professor 

 H. S. Jennings (a brilliant predecessor of mine in the 

 Vanuxem Lectures) with his fellow workers has investigated 

 the mating habits of a very well known one-celled animal, 

 Paramecium. They have discovered the remarkable fact that 

 two Paramecia of any one clone will not conjugate with each 

 other. The animal must find a mate not closely related to it. 

 By studying an immense number of animals of the species 

 Paramecium hursaria the investigators found that the whole 

 population of the species is distributed into several "mating 

 types" such that an individual of one type will mate with 

 one of another type, but not with one of its own. 



Conjugation is therefore a kind of outbreeding. To use a 

 figure of speech taken from higher animals, it introduces 

 "new blood" into the family, which causes an internal re- 

 arrangement of the cell materials and gives the race a new 

 start. The "mating types," it will be noted, are not in any 

 strict sense different sexes. As Jennings points out, in one of 

 his examples, types A and B will mate together and type C 

 will mate with either of them. Such an observation shows that 

 type C is not of a fixed "sex." The situation indeed is one of 

 relative sexuality, such as we cited above when we were trying 

 to imagine other ways of reproduction Nature might have 

 tried rather than the bisexual method that universally char- 

 acterizes higher animals. 



In some other one-celled animals, however, there seem to 

 be only two mating types, and in many species the two conju- 

 gating types are actually somewhat different in appearance. 

 This is getting closer and closer to bisexuality in the strict 

 sense. 



It is probable that conjugation or something like it goes 

 on in every kind of animal, although the details are not 



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