STRUCTURE AND PHYSIOLOGY OF ANIMALS 21 



operative between the male and female cells. In many instances 

 a prominence (the receptive prominence) is pushed out by the 

 ovum at the point where the sperm enters. The female 

 pronucleus, leaving its former central position, approaches the 

 male cell as it enters. In most cases a single sperm alone enters 

 the ovum in impregnation. According to the older observers, 

 as soon as a sperm enters the ovum, a membrane is formed 

 around the latter hindering the penetration of additional sperms. 

 But it has now been shown that such a membrane occurs 

 only in certain cases, and is quite exceptional. That, 

 as a general rule, only one sperm penetrates into the ovum 

 appears to be due to the circumstance that, as a result of the 

 entry of the one sperm, the peculiar attraction above referred 

 to becomes in some way destroyed or diminished. But, though 

 the entry of one sperm only is usual, cases of the entry 

 of several polyspermy, as it is termed are by no means 

 rare, and would appear to be quite normal in some groups of 

 animals. 



In some animals the ovum develops parthenogenetically i.e. 

 without any process of fertilisation by means of a male cell. 

 This is a normal phenomenon in certain families of insects, 

 for example. In a considerable number of marine invertebrate 

 animals it has been shown that -though gamogenesis, i.e. develop- 

 ment as the result of fertilisation of ovum by male cell, is 

 the normal process, yet parthenogenesis can be produced by 

 various artificial means. By adding various salts to the water 

 in which the ova are contained, by changes of temperature, 

 or by subjection to the action of carbonic acid gas, the ova, 

 in the absence of sperms, may be caused to give rise to normal 

 embryos. Such experiments on artificial parthenogenesis, as it 

 is termed, show that the entry of a male cell into the ovum 

 is not necessary for the development of the embryo even in 

 cases in which gamogenesis is normal ; but that other exciting 

 influences may bring about the same result. 



Though, as stated above, the female pronucleus, under normal 

 circumstances, plays so important a role in the development, it 

 has been shown that it can be dispensed with. When unfertilised 

 ova of a sea-urchin are broken up, and fragments devoid of 

 nuclei are placed in water along with sperms, the fragments may 

 be fertilised ; and, the nucleus of the sperm taking the place 

 of the segmentation-nucleus, normal young, differing from those 

 produced in the usual manner only in their smaller size, may 

 be developed. This phenomenon is known as merogony. 



The result of fertilisation is the formation of the impregnated 

 ovum, or oosperm as it is called. The oosperm, it is to be noted, 

 before development begins, consists in general of the primary 

 ovum minus the portions of the substance of its nucleus removed 



