296 THE PHYSIOLOGY OF REPRODUCTION 



high. Lutem is being formed actively iii the liver, which is yellow. 

 But the lutein does not appear in the blood, probably because the 

 sacculina roots absorb it. 



This production in the male crab of a metabolism of the female 

 type by infection with Sacculina may not stop merely at the appear- 

 ance of female secondary sexual characters in the male, but may go 

 so far that ova are produced in the testes. 



These remarkable facts have led Doncaster^ to assume that all 

 individuals contain potentially the characters for both sexes. — are, in 

 fact, potential hermaphrodites. Sex is determined by an additional 

 factor which suppresses the characters of one sex and causes those of 

 the other sex to appear. The sex-detei'mining factor does not 

 introduce the characters of T;he corresponding sex, but merely 

 releases them. And it does this by determming a certain type of 

 metabolism. In other words, metabolism determines sex, not sex 

 metabohsm. 



The Male Generative Okgans 

 The Semen 



The semen, i.e. the fluid discharged by an ejaculation, is the 

 secretory product of the testis, epididymis, vesiculse seminales, 

 prostate and Littrd's gland. In man it is a thick, viscous, yellowish, 

 opalescent fluid, which after ejaculation solidifies at first and after- 

 wards becomes fluid again. It has a peculiar smell, which becomes 

 even more noticeable on heating. Its reaction is alkaline. Its 

 specific gravity lies between 1-02 and 1'04. The amount discharged 

 in an ejaculation is given differently by different authors, and 

 probably varies with different individuals, and even with the same 

 individual at different times. From the figures given in the literature 

 5 gm. may be taken to be the average amomit.^ In a normal 

 emission of man Lode calculated that there are about 226 million 

 spermatozoa, and that about 340,000 million must be produced in an 

 individual between the ages of twenty-five and fifty-five. 



The different classes of animals show great differences in the 

 volume of semen ejaculated and in the relative proportion between 

 the spermatozoa and the liquid part of the semen in which they are 

 suspended.^ The semen of different animals also differs in its 



' Doncaster, "The Physiology of Sex Determination," British Assoc, 1913, 

 p. 671. 



2 Acton, Functions qnd Disorders of the Beproduotive Orgaiis, 3rd Edition, 

 London, 1862. Lode, " Untersuchungen tiber die Zahlen und Eegenerations 

 Verhaltnisse der Spermatozoiden bei Hund und Mensch," Pfliiger's Arch., vol. i., 

 1891. Mantegazza, Oaz. Med. Ital., Lombardia, 1866, quoted from Lode. 



^ Iwanoff, "LeSperme de quelques Mammif feres," Compt. Rend. Socde Biol., 

 vol. Ixxx., 1917. 



