296 THE PHYSIOLOGY OF REPRODUCTION 



high. Lutein is being formed actively in 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 1 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-determining factor does not 

 introduce the characters of the corresponding sex, but merely 

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

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



metabolism. 



^ 



THE MALE GENERATIVE OKGAXS 

 The Semen 



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

 secretory product of the testis, epididymis, vesiculse seminales, 

 prostate and Littre"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 T02 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 gin. may be taken to be the average amount. 2 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. 2 The semen of different animals also differs in its 



1 Doncaster, "The Physiology of Sex Determination," British Assoc., 1913, 

 p. 671. 



2 Acton, Functions and Disorders of the Reproductive Organs, 3rd Edition, 

 London, 1862. Lode, " Untersuchungen iiber die Zahlen und Regenerations 

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

 1891. Mantegazza, Gaz. Med. ItaL, Lombardia, 1866, quoted from Lode. 



3 Iwanoff, " Le Sperme de quelques Mammiferes," Compt. Rend. >SV. de 

 vol. Ixxx., 1917. 



