BEHAVIOR OF THE GAMETES 253 



diplotene state thus passes into the diakinesis stage (figs. 126C; 135 A, show 

 the breakdown of the nuclear wall and appearance of chromosomes in the 

 pigeon). 



The first maturation division occurs and the first polar body is extruded 

 shortly before ovulation (fig. 126D). The second maturation spindle is then 

 formed. In this state the egg is ovulated. From four to six sperm penetrate 

 into the egg shortly after it enters the infundibulum of the oviduct. The latter 

 events are consummated within fifteen minutes after ovulation. The second 

 maturation division then occurs, followed by the discharge of the second polo- 

 cyte, which becomes manifest about the time of, or shortly before, the fusion 

 of a single male pronucleus with the female pronucleus (fig. 126E). Thus, 

 although polyspermy is the rule, only one sperm pronucleus takes part in the 

 syngamic process. 



After the two pronuclei become closely associated, the chromosomes be- 

 come evident, the nuclear membranes disintegrate, and the first cleavage spindle 

 is formed in about five and one-quarter hours after the sperm enters the egg 

 (Olsen, '42). 



In the egg of the pigeon, according to Harper ('04), the germinal vesicle 

 breaks down, and the first polar spindle forms in the egg just before ovulation 

 (fig. 135A, B). Fertilization then occurs just as the egg (in reality the primary 

 oocyte) enters the oviduct. Normally from 15 to 20 sperm enter the blastodisc 

 of the pigeon's egg. However, only one sperm pronucleus associates with the 

 female pronucleus. Consequently, unlike the condition in the hen's egg, both 

 maturation divisions occur and the first and second polar bodies are given 

 off after sperm entrance (fig. 135C, D). Following the maturation divisions, 

 the two pronuclei proceed to associate (fig. 135E, F). The first cleavage 

 nucleus is shown in fig. 135G with two accessory sperm nuclei shown to the 

 extreme left of the figure. 



6) Fertilization in the Rabbit. In the rabbit, ovulation occurs around 10 

 to 1 1 hours after copulation. It takes about four hours for the sperm to travel 

 to the upper parts of the Fallopian tube. (See Chap. 4.) The sperm thus lie 

 waiting for about six to seven hours before the eggs are ovulated. When the 

 eggs are discharged frorii the ovary, each egg is surrounded by its cumulus 

 cells. The latter form the corona radiata, surrounding the zona pellucida 

 (fig. 124A). As the eggs are discharged from their follicles, an albuminous 

 substance from the follicles forms a clot, and several eggs are included within 

 this clot (fig. 124C). A sperm, therefore, must make its way through the sub- 

 stance of the clot, as well as between the cells of the corona radiata, and then 

 through the zona pellucida to reach the egg. This feat is accomplished partly 

 by its own swimming efforts and partly also by means of an enzyme (or 

 enzymes) which dissolves a pathway for the sperm. (See hyaluronidase, etc., 

 mentioned on pp. 229.) The ferment hyaluronidase, associated with the sperm, 

 frees the eggs from the albuminous clot and aids in the dissolution of the 



