412 THE MAINTENANCE OF SPECIES 



nuclear material, and finally separate. If the conjugating forms are 

 of equal size, as in the case of Paramecium, both usually survive and 

 continue to reproduce, by asexual means. On the other hand, when 

 the conjugants are of unequal size it frequently happens that the 

 smaller, or micro-conjugant, degenerates soon after conjugation. 



Other Invertebrates 



As division of labor among the cells of an organism progresses there 

 is increasing evidence of a gradual but none the less clear demarcation 

 into two sorts of cells, the soma or body cells, and the germ or sex cells. 

 These groups are separated early in the development of the individual, 

 the former being burdened with the responsibilities of movement, 

 protection, securing food, and in some cases caring for the young. 

 The second, comprising the germ cells, is solely concerned with the 

 elaboration of highly specialized cells adapted for the production of 

 new individuals, and so serving for the maintenance of the race. 



Since sexual reproduction undergoes many modifications in the 

 invertebrates, it appears logical to consider some of these phenomena 

 before undertaking a detailed study of sexual reproduction in the 

 higher vertebrates. 



Hermaphroditism 



Many of the lower invertebrates exhibit a kind of sexual reproduc- 

 tion in which both the male and female organs are found in the same 

 individual. A complete set of male and female reproductive organs 

 occurs, for example, in a single Hydra. In this genus the syermary 

 producing the spermatozoa is situated closer to the tentacular region 

 than the ovary which is located near the foot. These gonads rupture 

 when mature, and one of the liberated spermatozoa finally fertilizes 

 the ovum contained in a disrupted ovary. When both gonads are 

 functional on the same individual self-fertilization may occur. 



The earthworm likewise contains a complete set of male and female 

 reproductive organs in the same individual, but here, as in many of 

 the trematode flatworms, copulation takes place between two separate 

 individuals. In such cases the exchange of spermatozoa results in 

 cross-fertilization. 



While hermaphroditism is unusual in the vertebrates, it is believed 

 to occur normally in a few instances such as certain hagfishes (cyclo- 

 stomes) which are known to be hermaphroditic. In these forms, 



