112 



FERTILIZATION 



whether an egg is polyspermic or monospermic. Fig. 22 (Roths- 

 child & Swann, 1952) shows the sort of curve obtained when this 

 experiment is done. In fourteen experiments of this type, the 

 average figure for the conduction time of the block to polyspermy 

 was 63 seconds. This type of experiment not only enables one to 

 measure the time taken for the egg to become completely imperme- 

 able to spermatozoa after fertilization ; it also enables estimates to 



0-75 



V) 



.y 0-5 



VI 



o 



o 



c 

 o 



0-25 



o 

 o 



^ — 9 • 



510 20 40 50 65 



T 100105 



150 



200 



FIC 



tfsec.) 

 22. — Proportion of polyspermic eggs {Psammechinus miliaris) in a suspension 

 after various times of contact between eggs fertilized at / = o, and spermato- 

 zoa. Sperm density, g- 1 1 < 10' /ml. #, experimental points; O, theoretical 

 points. T, 85 sec. The thick line from 65-105 sec. on the time axis is the 

 interval within which T lies with a fiducial probability of 09 (Rothschild & 

 Swann, 1952). 



be made of a during the passage of the 63-second change. During 

 this time, a is only i/20th of what it is before the first fertilization, 

 which means that the receptivity of the egg surface is twenty times 

 higher before than after the first fertilization. After the 63 seconds, 

 the receptivity of the egg surface is zero. The implications of these 

 observations are shown in Fig. 23, from which it will be seen that 

 the block to polyspermy is probably diphasic, in the sense that a 

 partial block to polyspermy sweeps over the egg surface in a 

 second or so, and is followed by a slower mopping-up process 

 which makes the egg completely impermeable to spermatozoa. 



