52 The Developmenf of the Vertebrate Embryo 



Sperm are generally just as short-lived as eggs. However, in some 

 cases ( bees, bats ) the sperm can remain viable for years within the female 

 genital tract. The study of sperm preservation has recently gained great 

 economic importance because of the use of artificial insemination for 

 livestock breeding. In the case of cattle, a stud bull is induced to service 

 an artificial vagina ( a water-jacketed rubber tube with a collecting bottle 

 at one end and the bull at the other). About 4.5 ml of semen is collected 

 per ejaculation, and the bull can be used for service up to 10 times in a 

 two-hour period. The semen is cooled slowly to a temperature just above 

 freezing and can be stored for seven days at most. It is diluted before use 

 and injected into the uterus of a cow in heat, through a pump. A good 

 stud bull can be used to impregnate literally thousands of cows in a single 

 season. It should also be noted that many human females fail to conceive 

 by natural means even though the gametes of both husband and wife are 

 perfectly viable. In such cases, artificial insemination of the wife with her 

 mate's semen is generally successful in inducing pregnancy. 



FERTILIZATION 



The act of fertilization can be divided arbitrarily into three phases: 

 ( 1 ) penetration of the egg by the sperm, ( 2 ) activation of the egg, and 

 (3) the union of egg and sperm nuclei. 



PENETRATION. Coutact between sperm and egg is generally the result 

 of random collision. The discharge of huge numbers of sperm by the male 

 increases the chance that at least one such collision will occur. Upon con- 

 tact, the sperm is bound tightly to the outer surface of the egg. This may 

 be accomplished by chemical reactions between proteins at the surfaces of 

 both gametes called fertilizins. Fertilizin, isolated from eggs, causes sperm 

 to become sticky and to clump together, while anti-fertilizin obtained 

 from the sperm does the same thing to eggs. The substances are specific 

 and act only upon gametes of the same species. Thus, fertilizin and anti- 

 fertihzin may increase the chance of an effective and lasting contact be- 

 tween egg and sperm. However, the evidence that the fertilizins actually 

 do operate during fertilization is still equivocal. 



The sperm that is now bound to the egg must penetrate its mem- 

 brane. Material extracted from sperm has been shown capable of dis- 

 solving the egg membrane by enzymatic action. Presumably the intact 

 sperm employs the enzyme to dissolve a hole big enough to permit en- 

 trance. In mammals, the egg is surrounded by a layer of tiny follicle cells 

 cemented together by a substance called hyaluronic acid. Mammalian 

 semen contains an enzyme, hyaluronidase, that dissolves this cement and 

 permits the sperm to slip through. Many pathogenic bacteria use this same 

 device to slip through body tissues and invade all parts of the organism. 



