ORIGIN OF THE INDIVIDUAL 241 



jelly-like layer is formed about the egg, or, if a membrane 

 is already present, this may be rendered impermeable or 

 still another formed. In cases where the egg is surrounded 

 originally by a dense and resistant wall, the tiny opening 

 provided for the entrance of the sperm is closed. How- 

 ever, the accessory wrappings about certain eggs, such as 

 those of Birds, have no relation to the present subject, 

 since they are secreted, not by the egg itself, but by glands 

 in the wall of the oviduct some time after fertilization has 

 occurred. 



The reactions of the egg cytoplasm that exclude accessory 

 sperm are overshadowed in importance by others which upset 

 the stable equilibrium of the egg and render its surface 

 permeable, so that extensive osmotic interchanges take place 

 between the cytoplasm of the egg and its surroundings. Most 

 often this is visible merely in a shrinkage of the cytoplasm 

 due to loss of water, but sometimes contractions, amoeboid 

 movements, or flowing of special cytoplasmic materials to 

 definite regions of the egg are visible. In any event it is cer- 

 tain that the cytoplasm undergoes profound changes 

 its organization as a gamete gives place to a reorganization 

 which establishes and determines the general outlines of its 

 subsequent development as an individual. (Fig. 132, A, B.) 



Turning now to the nuclei, known as male and female 

 PRONUCLEI, the union of which to form the single fertiliza- 

 tion nucleus (SYNKARYON) is the climax of fertilization. Dis- 

 regarding the flagellum of the sperm, which disappears as it 

 enters the egg, we find that the sperm nucleus moves through 

 a quite definite path toward the center of the egg where it is 

 met by the egg nucleus. Both the pronuclei now become re- 

 solved into chromosomes which lie free in the cytoplasm, 

 while a pair of centrosomes, surrounded by asters, appear and 

 take up positions on either side of the chromosomes to form a 



