I GROWTH OF THE GERM-CELLS 27 



where the axis with unlike poles (animal and vegetative) is 

 determined by the elongation of the ovarian egg, the forma- 

 tion of the micropyle, and liberation of the contents of the 

 germinal vesicle into the cytoplasm. All telolecithal eggs 

 (e. g. Vertebrate eggs) have a similar polar structure similarly 

 determined by the disposition of yolk, and sometimes by the 

 presence (Frog) of pigment as well, and by the position of the 

 nucleus. Such an egg is divisible into similar halves by any 

 plane which includes the axis. But the egg may be centro- 

 lecithal (most Coelenterates), and here the axis is only deter- 

 minable by the excentricity of the nucleus, though in one 

 interesting case (Carmarina) there is an excentric jelly-plasm 

 near the vegetative pole. In some cases ^Cephalopoda, Insects) 

 the egg is bilateral. 



When the germ-cells meet and unite in the act of fertiliza- 

 tion the full number of chromosomes (2 n) is of course restored, 

 but that is not the only nor even the chief event involved in 

 the process. In fertilization four distinct events occur, and some- 

 times a fifth. The first of these is the extrusion by the ovum 

 of a fluid, the perivitelline fluid ; the second is the entrance 

 of the spermatozoon ; the third is the appearance of the defini- 

 tive centrosome and its division into two to form the cleavage 

 apparatus of asters and spindle ; the fourth is the union of the 

 male and female pronuclei. To these must be added, in some 

 cases at least, a fifth, the alteration of the structure and 

 symmetry of the egg. 



1. The moment a spermatozoon comes in contact (by its 

 acrosome) with the surface of the egg the latter extrudes a 

 fluid, termed perivitelline. This process is often accompanied 

 by the formation of a membrane, the fertilization membrane, 

 as in the Sea-urchin, which prevents the entrance of more 

 spermatozoa. This membrane, in the Sea-urchin, is not pre- 

 formed, but is to be regarded as the surface-layer of the egg 

 itself, gradually lifted up and separated from the egg by the 

 collection beneath it of a fluid the perivitelline fluid which 

 is hypertonic to sea- water. The membrane so formed quickly 

 becomes spherical. 



In the Polychaet worm (Nereis) the mechanism is rather 



