FERTILISATION 



183 



may effect an entrance an y where on the surface (some Echinoderms 

 and Ccelenterates), or there may be funnel-shaped depressions on the 

 egg's periphery (certain Hydromedusse). 1 



In the majority of animals only one spermatozoon normally enters 

 the ovum, but in some (certain insects, Elasmobranch fishes, reptiles, 

 earthworm, lamprey, axolotl, 2 etc.), 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 the hen's egg polyspermy would seem to be the normal 

 condition, as five to twenty-five 

 sperm have been observed in a 

 single ovum by Patterson. 3 While 

 only one spermatozoon unites with 

 the egg nucleus the supernumerary 

 ones distribute themselves through- 

 out the margin of the blastodisc in 

 the later phases of fertilisation and 

 the first stages of segmentation. 

 They undergo a certain amount of 

 division, forming small groups of 

 daughter nuclei, and these divisions 

 are frequently accompanied by 

 slight cleavage of the surrounding 

 cytoplasm of the margin of the 

 blastodisc, forming the accessory 

 cleavage planes. At a later period, 

 when the egg has reached the 

 32-cell stage, they have usually 

 undergone degeneration and have disappeared. 



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 eggs 

 which are fertilised in this way either do not divide at all or go on 

 dividing irregularly for a short time and then perish. 4 



A point of considerable interest is that, in some animals, the 

 entry of the sperm into the egg-cell takes place very early, the 

 sperm remaining passive in the cytoplasm of the egg throughout 



1 Wilson (E. B.), The Cell, etc., 2nd Edition, New York, 1900. 



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



3 Patterson (J. T.), " Studies on the Early Development of the Hen's Egg," 

 Jour. Morpk., vol. xxi., 1910. 



4 Wilson, /o<: cit. 



FIG. 58. Fertilisation process in 

 bat's ovuin. (After van der 

 Stricht.) 



p.b., Polar bodies ; .., nucleus 

 of ovum ; $.., nucleus of 

 spermatozoon. 



