BEHAVIOR OF THE GAMETES 243 



gatus, a protoplasmic prominence appears only after a sperm begins to pass 

 into the egg. It persists until about the time that the pronuclei unite (Wilson 

 and Mathews, 1895). (See fig. 131B-F.) A prominent fertilization cone is 

 found also in the starfish, Asterias jorbesi (Wilson and Mathews, 1895). 

 In the vertebrate group, fertilization cones are not generally observed, but 

 the protoplasmic bridge from the egg membrane to the ooplasmic surface in 

 Petromyzon evidently fulfills the functions of a cone (fig. 134C). 



The formation of the fertilization cone and its withdrawal again, suggests 

 that ooplasmic movements are concerned mainly with the sperm's entry into 

 the interior of the egg. These movements appear to be aroused by some stimulus 

 emanating from the sperm as it contacts the egg's surface. That is to say, al- 

 though the sperm becomes immobile once it has touched the egg's surface, 

 various stimuli, chemical and/or physical, issue from the sperm into the egg 

 substance. Here these stimuli inaugurate movements in the ooplasm which draw 

 the sperm into the egg. This modern view thus emphasizes motility of the 

 cortical area of the egg as the factor which conveys the sperm into the interior 

 of the egg. It suggests further that the older view of sperm entry which was 

 presumed to result from sperm motility alone does not agree with the actual 

 facts demonstrated by observation. 



c. Some Changes in the Physiological Activities of the Egg at Fertilization 



The separation of the egg membrane from the egg surface, the emission 

 of fluid substances from the egg's surface into the perivitelline space, and 

 contraction of the egg's surface have been noted above. Associated with these 

 immediate results of sperm contact with the egg, a pronounced movement 

 of cytoplasmic substances within the egg can be demonstrated in many species. 

 Examples of cytoplasmic movements within the ooplasm of the egg are given 

 below in the descriptions of the fertilization processes which occur in various 

 chordate species. 



Accompanying the above-mentioned activities, pronounced changes of a 

 metabolic nature occur. In the egg of the frog and toad, for example, there 

 is little change in the oxygen consumption during fertilization, although there 



Fig. 121. Two stages of fertilization in animals. (A) In the primary phase of fer- 

 tilization ("external fertilization" of F. R. Lillie), the sperm is activated to greater 

 motility by the environmental factors encountered at the fertilization site, including the 

 gynogamic substances secreted by the egg. It is also drawn to the egg by a positive 

 chemotaxis. The lytic substances (androgamic substances) enable the stimulated sperm 

 to make its way more easily through the jelly membranes and ooplasmic membranes 

 surrounding the egg to the egg's surface. At the egg's surface the interaction of gynogamic 

 and androgamic substances brings about the agglutination of the sperm to the egg's 

 surface. This initiates stage B, on the secondary phase of the fertilization process ("in- 

 ternal fertilization" of F. R. Lillie). (B) Secondary phase of fertilization or fusion of 

 the gametes. (See text for further description.) This stage begins when the sperm has 

 made contact with the egg and terminates when the first cleavage spindle has formed. 



