V CROSS-FERTILISATION 339 



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 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 302). 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 fertilization does not occur owing 

 to contrivances of various kinds for its prevention. It has 

 been proved in numerous instances that cross-fertilization — 

 t'.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 genera- 

 tion. 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-impregnation 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 fertilized 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 sur- 

 faces and attached to one another in this position by a 

 viscid secretion from the clitellum. The sperms are then 

 passed from the male apertures of one into the sperm othecK of 

 the other individual, and the two worms afterwards separate. 

 The clitellum then secretes a tough chitinous tube or cocoon 



z 2 



