190 THE PHYSIOLOGY OF REPRODUCTION 



normally after the addition of spermatozoa, while Driesch 

 has shown that if such ova are deprived of their envelopes 

 by shaking, and are then divided into fragments some of 

 which contain no nuclei, the latter are capable of being ferti- 

 lised a second time. It is clear, therefore, that in such cases 

 the union of nuclei is not essential for the development of the 

 ovum. 1 



In those ova which are surrounded by a membrane it is 

 probable that the fertilising spermatozoon bores its way through 



at any point (Mammals and Am- 

 phibians). In other cases there is a 

 small aperture in the wall of the 

 ovum ; this is called the micropyle 

 (some Pisces and Insecta). Some 

 eggs, however, are naked, so that 

 the sperms may effect an entrance 

 anywhere on the surface (some 

 Echinoderms and Coelenterates), or 

 there may be funnel-shaped depres- 

 sions on the egg's periphery (certain 

 hydromedusse). 2 

 FIG. 52. Fertilisation process In the majority of animals only 



in bat's ovum. (After van one spermatozoon normally enters 



the ovum, but in some (certain 

 p. b , P olar bodies ; o. n., nucleus insects e lasmobranch fishes, reptiles, 



of ovum: s. n., nucleus of , 



spermatozoon. earthworm, lamprey, axolotl/ &c.), 



several may effect an entrance. 



The latter condition is called Polyspermy. Only one sperm- 

 nucleus conjugates with the ovum-nucleus ; the others as 

 a general rule undergo degeneration, but in a few cases 

 (elasmobranchs and reptiles) they are said to divide, forming 

 accessory nuclei whose ultimate fate is unknown. In those 

 animals in which only one sperm normally enters the egg, 

 pathological polyspermy may occasionally occur. In such cases 

 each sperm centrosome may give rise to a sperm-aster. The 



1 For references to the original papers, which are somewhat numerous, 

 see Przibram, Embryogeny, English Translation, Cambridge, 1908. 



2 Wilson, The Cell, &c., 2nd Edition, New York, 1900. 



3 Jenkinson, loc. cit. Further references are given in this paper. 



