BIOLOGY OF SPERMATOZOA 



727 



(Icinonstrated in the bull an abundant 

 sperm-free initial portion which apparently 

 is derived from the urethral glands and pre- 

 sumably serves to clear the urethral passage 

 b(>fore the transport of spermatozoa (Lut- 

 wak-jNIann and Rowson, 1953). In man, 

 however, approximately three fourths of the 

 sjierm are present in the first 40 per cent of 

 the ejaculate (MacLeod and Hotchkiss, 

 1 942 ) . Qualitative contributions to the total 

 ejaculate by the several accessory glands 

 liave been carefully studied and are dis- 

 cussed elsewhere (see chapter by Price and 

 Williams- Ashman ) . 



D. EFFECTIVE SPERM CONCENTRATION 



It is not a simple matter to determine 

 what might be the minimal effective sperm 

 count necessary to insure fertilization. The 

 earlier standards of what constitutes a sub- 

 fertile human seminal density have under- 

 gone considerable re-evaluation. The once- 

 acceptable value of minimal concentration, 

 80 to 100 million cells per ml. of semen, has 

 now been reduced to one half or less. On the 

 other hand, the spotty records of pregnan- 

 cies in women whose husbands' sperm counts 

 consistently average 1,000,000 per ml., or 

 less, may be viewed with some skepticism 

 CMichelson, 1951; Sandler, 1952). The ex- 

 tensive studies of ]\IacLeod and Gold (1951) 

 on human subjects of proved fertility, com- 

 pared with men of infertile marriages, indi- 

 cate a significant break between the two 

 groups in the neighborhood of 20,000,000 

 cells per ml. of semen. Current trends in the 

 evaluation of semen tend to minimize sperm 

 density, as such, and to regard this prop- 

 erty only with reference to other criteria, in- 

 cluding volume, total sperm number, mor- 

 phology, and, of course, motility. 



A reasonable gauge of minimal effective 

 si)erm count necessary to insure fertilization 

 lias been provided by dilution tests and arti- 

 ficial insemination of domestic and labora- 

 tory animals. In cattle, the normal ejacu- 

 late, which contains some 4 billion sperm, 

 can be reduced 500 to 1000 times without 

 sacrificing high productivity (Salisbury and 

 Bratton, 1948; Braden and Austin, 1953). 

 Ral)l)it fertilization is unimpaired when the 

 normal inseminate is decreased 500-fold 

 (Cheng and Casida, 1948; Chang, 1951a; 

 Chang, 1959; Braden and Austin, 1953). 



That mere number of sperm is not the only 

 factor was clarified by Chang (1946a, b) 

 who showed that the concentration and na- 

 ture of the diluent are also important. The 

 percentage of fertile eggs recovered from 

 does inseminated with a suboptimal num- 

 ber of sperm (ca. 40,000) decreases as a 

 function of the volume of saline diluent 

 (from 0.1 to 1.0 ml.). On the other hand, if 

 rabbit seminal plasma is substituted for sa- 

 line as the diluent, fertilizing capacity is 

 enhanced (Chang, 1947b, 1949). The nature 

 and the effect of the sperm diluent are fur- 

 ther discussed below ; it is sufficient to point 

 out here that many factors may determine 

 the absolute number of sperm required for 

 a high rate of fertilization. 



E. SITE OF INSEMINATION 



The location of the deposition of semen 

 during ejaculation differs in various animals 

 and may account, in part, for the variations 

 recorded for time of transport through the 

 female genital tract. Intravaginal insemina- 

 tion predominates in the rabbit, dog, ewe, 

 cow, and man, whereas intrauterine depo- 

 sition occurs in the mouse, rat, sow, mare, 

 and probably the hamster (Braden and Aus- 

 tin, 1953; du Mesnil du Buisson and Dau- 

 zier, 1955; Chang and Sheaffer, 1957). 



Experimental insemination has been at- 

 tempted by a number of routes. Administra- 

 tion of sperm into the ovarian bursa of re- 

 ceptive mice proved highly satisfactory 

 (Runner, 1947). Intraperitoneal insemina- 

 tion has been accomplished in fowl (Van 

 Drimmelen, 1945), guinea pigs (Rowlands, 

 1957), rabbits (Hadek, 1958b), and, with 

 bare success, in the cow (Skjerven, 1955; 

 McDonald and Sampson, 1957). In an ex- 

 tensive animal breeding investigation, Salis- 

 bury and VanDemark (1951) showed that 

 artificial insemination was equally effective 

 in cattle, as judged from the nonreturn rate, 

 when semen was deposited in the vagina, the 

 body of the uterus, or the uterine horns. 



F. .\RTIFICIAL INSEMIN.\TION 



Little more than a brief account of this 

 special and applied subject seems appropri- 

 ate at the moment. Excellent surveys of the 

 development, techniques, and accomplish- 

 ments of artificial insemination have ap- 

 peared from time to time, two of the more 



