28 The Biochemistry of Semen 



Coagulation and liquefaction 



Semen is ejaculated in a liquid or semi-liquid form. In some animal 

 species such as the bull and the dog, it remains liquid, but in others 

 it tends to gelate or coagulate shortly after ejaculation. Human 

 semen clots immediately after ejaculation only to liquefy again a 

 little later; until that happens the spermatozoa do not become fully 

 motile. For this reason the examination of human spermatozoa 

 should be postponed until at least twenty minutes after the emission 

 of semen (McLeod, 1946<2; Sunmons, 1946). Quite fresh boar semen 

 usually contains only small lumps of gelatinous material somewhat 

 resembling tapioca; on standing, however, the lumps increase and 

 merge into a semi-solid gelatinous mass which may take up half or 

 more of the entire ejaculate. Gelation of semen can also be observed 

 in the stallion. Even more striking is the clotting phenomenon in 

 the rodents. The major part of a rabbit ejaculate collected by means 

 of an artificial vagina often consists of a colourless transparent gel. 

 In rats and guinea-pigs, semen coagulation leads to the formation 

 after mating, of the so-called bouchon vaginal or vaginal plug, which 

 probably prevents the back-flow of semen from the vagina and 

 assists the passage of spermatozoa through the cervix into the uterus. 

 In a study of sperm transport in the rat, Blandau (1945) has shown 

 that the ejaculate fails to pass through the uterine cervix if the 

 coagulation of semen is aboUshed by ligation of the seminal vesicle 

 and coagulating gland ducts. The copulatory plug has also been 

 described in Insectivora (mole and hedgehog), Chiroptera {Rhinolo- 

 phidae and Vespertilionidae) and in Marsupialia (Camus and Gley, 

 1899; Courrier, 1927; Eadie, 1939, 1948a, b; Engle, 1926; Rollinat 

 and Troussart, 1897; Stockard and Papanicolaou, 1919). In most 

 animals the vaginal plug is due to the clotting of the semen itself but 

 in some, namely in the opossum (Hartman, 1924) and in the bat 

 Vesperuga noctula (Courrier, 1925; Grosser, 1903), its occurrence 

 involves the coagulation of female secretions by the seminal plasma. 

 In the honey bee, the escape of semen from the female reproductive 

 tract is prevented by the so-called mucus plug; this is formed by the 

 material ejected from the mucus glands of the drone towards the end 

 of ejaculation (Laidlaw, 1944). 



In most species the substrate for gel formation consists of protein- 



