6o THE EVOLUTION OF SEX. 



goes further than the setting apart of special organs. Entire 

 individuals become reproductive "persons" (as they are techni- 

 cally called), in contrast to the nutritive persons of the colony. 

 {b) In some of those reproductive individuals, a curious 

 phenomenon, known as migration of cells, has been observed 

 by Weismann and others. The reproductive cells, arising in 

 various parts of the body, have been shown to migrate in some 

 cases to another part, where they find final lodgment in more 

 or less definite organs. This occurrence is intimately associated 

 with "alternation of generation," and will be afterwards discussed 

 under that heading. 



It is in nowise the purpose of the present work to describe 

 the details respecting the ovaries and testes, as they occur in 

 the various classes of animals. It is enough for our purpose to 

 have emphasised the fact of their gradual differentiation, and to 

 note that they are almost always developed in association with 

 the middle layer of the body, and usually occupy a posterior 

 position on the wall of the body-cavity. The details will be 

 found in any standard work on comparative anatomy, very con- 

 veniently for example in Prof. Jeffrey Bell's " Comparative 

 Anatomy and Physiology," London, 1885. 



§ 2. Associated Dticts. — It is only in a few animals, like hydra and its 

 allies, that the ovaries and testes are external organs, which have simply to 

 burst to liberate their contents. They are usually of course internal, and thus 

 arises the necessity of some means of communication with the outside world. 

 In the simplest cases, the male elements find their way out to the sur- 

 rounding medium without any specialised mode of exit. They there meet, 

 by chance combined with physical attraction at short range, with the ova, 

 which in the simplest cases again have found their way out in an equally 

 primitive fashion. Thus in the enigmatical parasitic mesozoa (orlhonectids, 

 &c.), liberation of the germs may occur by perforation or by rupture of the 

 excessively simple bodies. In some of the marine worms {e.i^.^ Polygoj-dius\ 

 the liberation of the ova at least is accompanied by the fatal rupture of the 

 mother organism, a vivid instance of reproductive sacrifice. Even in some 

 of the common nereids, the same uneconomical mode of liberation by rupture 

 appears to occur. The forcible rupture may be referred to pressure of the 

 relatively large mass of growing cells which the ovaries often present. 



As high up as back-boned animals, the absence of ducts may be traced. 

 Thus among the sea-squirts or tunicates, the reproductive organs are fre- 

 quently ductless, and the same thing is true of some fishes. The sex-cells 

 burst into the body-cavity, and thence find their way to the exterior by aper- 

 tures. In most cases, where ducts are absent, fertilisation of the ova is 

 external, but this is not necessarily so. In sponges, for instance, fertilisa- 

 tion is almost always internal. Male elements are washed in by the water- 

 currents, find their way to the ova, and fertilise them /;/ situ. Almost 

 without exception, embryo-sponges, not ova, make their way to the exterior. 

 In the higher animals, where definite ducts are present, alike for the inward 



