BEHAVIOR OF THE GAMETES 255 



corona radiata cells, so that each egg lies free in the Fallopian tube, sur- 

 rounded by the zona pellucida. It may be that some other lytic substance asso- 

 ciated with the sperm also is active in aiding the sperm to reach the egg's 

 surface. 



The first maturation division of the egg occurs as the egg is being ovulated. 

 The egg remains in this condition until the sperm enters, which normally 

 occurs within two hours after ovulation. Thus, sperm entrance into the rabbit's 

 egg presumably is much slower than in the case of the hen's egg, possibly due 

 to the albuminous and cellular barriers mentioned above. Several sperm may 

 penetrate through the zona pellucida into the perivitelline space, but only one 

 succeeds in becoming attached to the egg's surface (Pincus, '39). The sperm 

 tail is left behind in the perivitelline fluid, and the sperm head and middle 

 piece "appear to be drawn into the egg cytoplasm rather rapidly" (Pincus 

 and Enzmann, '32). The second polar body is then extruded, a process which 

 ordinarily is completed about the thirteenth hour following copulation (fig. 

 124A). About three or four hours later (that is, about 17 hours after copu- 

 lation) the two pronuclei are formed and begin to approach one another, 

 and at 20 to 23 hours after copulation the pronuclei have expanded to full 

 size and come to lie side by side (fig. 124B). The migration of the pronuclei 

 to the center of the egg thus consumes about four to six hours. The spindle 

 for the first cleavage division generally is found from 21 to 24 hours after 

 copulation (Pincus, '39). (Consult also Gregory, '30; Lewis and Gregory, '29.) 



7) Fertilization in the Echidna, a Prototherian Mammal. The egg of the 

 Tasmanian anteater. Echidna, when it reaches the pouch is about 15 by 13 

 mm. in diameter. This measurement, of course, is only approximate, and it 

 includes the egg proper plus its external envelopes of albumen and the leathery 

 shell. (The egg of Ornithorhynchus is slightly larger, approximating 17 by 14 

 mm.) At the time of fertilization in the upper portion of the Fallopian tube, 

 the fresh ovum of Echidna without its external envelopes measures about four 

 to 4.5 mm. in diameter. 



The fully developed eggs of the monotreme (prototherian) mammals are 

 strongly telolecithal, with a small disc of true protoplasm situated at one pole 

 as in the bird or reptile egg. In Echidna aculeata this disc measures about 

 0.7 mm. in diameter during the maturation stages. Just before the onset of 

 the maturation divisions of the nucleus, the germinal vesicle is saucer-shaped 

 and lies in the midportion of the upper part of the disc (fig. 127A). The first 

 maturation division (fig. 127B, C) occurs before ovulation, while the second 

 maturation division (fig. 127D, E) occurs after ovulation. There is some evi- 

 dence that the second maturation division, in some cases, may precede the 

 actual entrance of the sperm into the germinal disc (Flynn and Hill, '39). In 

 figure 127F-H, the stages in the association of the male and female pronuclei 

 are shown. 



As fertilization is accomplished, a rearrangement and movement of the 



