VIABILITY OF EGGS AND SPERM 51 



All this is purely hypothetical, but in the case of the sea urchin there is 

 good visible evidence that something like this happens at or near the surface 

 of the egg. Just within the surface of the unfertilized egg there are a number 

 of fine granules. As one watches fertilization under the microscope, within 

 20 seconds after the sperm makes contact with the surface of the egg the 

 layer of surface granules completely fades (Fig. 22). The granules begin 

 to swell up and disappear, first at the point of entrance of the sperm, and 

 then in an ever-widening area beneath the egg surface. This is what can be 

 seen. Possibly this is what happens: Upon the entrance of the sperm, layer B 

 swells off the surface of particle A, and this swelling goes on around the 

 entire surface of the egg. A and C may now react, and development proceeds. 

 These same granules break down when the egg is stimulated chemically by 

 some parthenogenetic agents, and therefore there is a close correlation 

 between stimulation of the egg and the disappearance of these cortical 

 granules. 



Viability of eggs and sperm 



The length of life of an egg and the length of its fertilizability are very 

 variable in different species. The period during which an egg may be 

 fertilized after it is released from the ovary is difficult to ascertain in 

 mammals. Even if the exact time of ovulation and the time of copulation 

 are known, there is still the question of the time necessary for the sperm to 

 travel through the uterus and into the oviduct. In general, the mammalian 

 egg remains fertilizable for about 24 hours after ovulation. Some invertebrate 

 eggs, such as those of the sea urchin, retain the capacity for being fertilized 

 for about 48 hours. On the other hand, other invertebrate eggs must be 

 fertilized immediately after their release. 



It is relatively easier to determine the viability of sperm. First of all, the 

 length of life of a sperm is not an index of its fertilizing power. A sperm 

 may be perfectly active and apparently normal and yet not be able to fertilize 

 an egg. For example, sea urchin sperm will live for several days, but their 

 fertilizing power drops off rapidly; it is usually lost in about 12 hours. How- 

 ever, both the length of life of the sperm and their fertilizing power are 

 functions of a number of variables which have been fairly accurately worked 

 out. 



One variable is the concentration of the sperm. The sperm must be con- 



