348 THE EARTHWORM CHAP. 



The cells of which the spermaries are composed do 

 not develop into sperms in the testes themselves, 

 but pass into the sperm-sacs, where they undergo 

 division into rounded masses of cells (gametocytes, com- 

 pare p. 286) looking very much like a segmenting oosperm 

 in the polyplast stage. Each of these products of division 

 of the testicular cells becomes elongated, and gradually 

 takes on the form of a sperm with a rod-like head and a 

 vibratile tail (compare pp. 194 and 307). When set 

 free, the sperms pass into the spermiducts through the 

 ciliated funnels. 



It is well known that many flowers (the reproductive 

 organs of higher plants) contain the generative cells of 

 both sexes, enclosed within the ovules and anthers 

 respectively ; yet in very many cases self-fertilisation 

 does not occur owing to contrivances of various kinds 

 for its prevention. It has been proved in numerous 

 instances that cross-fertilisation i.e., the impregnation 

 of the ovum in one individual by the male cell of another 

 is of great importance in keeping up the strength 

 and vigour of the plant from generation to generation. 

 The same is true amongst animals ; and though in some 

 monoecious forms, such as the Hydra, there is no 

 special arrangement for the prevention of self-impreg- 

 nation if the male and female gametes of the same 

 individual ripen at the same time, in others, such as the 

 earthworm, the ova are always fertilised by the sperms 

 from another individual. 



This is effected in the earthworm in the following way. 

 Two individuals, their anterior ends pointing in opposite 

 directions, become applied together by their ventral 

 surfaces and attached to one another in this position 

 by a viscid secretion from the clitellum. The sperms 

 are then passed irom the male apertures of one 

 into the spermothecae of the other individual, and the 



