36 General Part. 



prevent the entrance of others ; in EcMnoderms, for instance, a delicate mem- 

 brane is seci-eted over the surface. In some cases, however, several sperm-cells; 

 penetrate the ovum {e.g., Squalidae), but only a single one unites with the female 

 pronucleus, the rest move about in the protoplasm for some time but eventually 

 disappear. 



For the phenomena in Protozoa corresponding to the fertilisation of Metozoa. 

 see Special Part. 



In many cases, most -Pish, Frogs, etc., fertilisation occurs in tlie 

 water outside the body of the parent. The female lays the eggs and 

 at the same time, or soon after, the male deposits the spermatozoa. 

 The genital products are mingled together, and the spermatozoa 

 have an opportunity of making their way into the ova. In other 

 cases fertilisation, takes place in the oviducts of the female, into 

 which the spermatozoa are introduced by the male copulatory organs, 

 (copulation) . 



Copulation is not always followed immediately by fertilisation. In many 

 animals spermatozoa may be retained within the body of the female for a long 

 time without losing their ef5cacy. (In Hens, two to three weeks ; Bats, as many 

 months ; Queen-bees, three years.) Frequently the ova are very immature when, 

 it occurs. 



As regards hermaphrodites, it is r^ry seldom that self -fertilisation 

 obtains, i.f., that an ovum and a spermatozoon from the same- 

 individual unite ; hermaphrodites almost always copulate, and then 

 fertilisation is either reciprocal, or one individual only, gives- 

 spermatozoa to the other. 



Hybridisation. — An ovum under ordinary circumstances is 

 fertilised by a spermatozoon which has been formed in an animal 

 of the same species. In most cases, however, it is possible for 

 ova to unite with spermatozoa of a nearly allied species; indeed, 

 copulation between two more distantly connected species may some-, 

 times occur.* As a rule, such a union results in a new organism, 

 a hybrid, which is more or less imperfect. For instance, 

 hybrids, even of nearly related species are very often sterile, 

 i.e., they are unable to produce ripe ova or sperinatozoa, although 

 they may be otherwise strong and well- developed animals, e.g., the 

 mule of the Horse and Ass. In other cases the hybrid is feeble and 

 dies young, or even does not outlive the embryonic period (hybrid 

 of pheasant and Domestic Fowl). On the other hand, however, 

 there are hybrids quite as well developed as the parent species, 

 and also perfectly fertile. This holds for the crosses between certain 

 species of Stags, different species of Pheasants, the hybrid of the- 

 Chinese and European Goose, &c. 



* It has been recently found by experiment, that Frog's spa-wu can be fertilised 

 by the spermatozoa of the Netrt, and the eggs of Eegular Sea-urchins by the sper- 

 matozoa of Irregular species ; but development is usually irregular and soon ceases. 

 On the other hand, there are nearly allied forms whose genital products will not 

 unite; and there are species which give remarkable results in that ova from one. A, can 

 be fertilised by spermatozoa from another, B, but not ova from B by spermatozoa. 

 from A. 



