GEXERATION OF ANIMALS, II. iv. 



sometimes, if the uterus happens to be in a suitable 

 condition and hot owing to the evacuation of the 

 menses, the uterus draws it in. Evidence for this is 

 the fact that pessaries though wet when applied are 

 dry when removed. Also, in those animals (such as 

 birds and \iviparous fishes) whose uterus is close by 

 the diaphragm there is no alternative : the semen 

 must be drawn in ; it cannot enter at the moment of 

 discharge. This region, in \irtue of the heat present 

 in it (the discharge and aggregation of the menstrual 

 fluid also produce fiery heat in this part) draws up the 

 semen in the same way that conical vessels which 

 have been washed out with something warm draw 

 water up into themselves when they are turned 

 mouth downwards. And that is the way in which 

 the semen is dra\\Ti in ; it is certainly not done, as 

 some allege, by the parts that are instrumental in 

 copulation." We find the situation reversed in the 

 theory that the woman as well as the man emits 

 semen, since if the uterus emits any semen outside 

 itself, it will have to draw it back inside again if it 

 is to mingle with the semen of the male. Such a 

 performance is superfluous, and Nature does nothing 

 which is superfluous. 



The action of the semen of the male in " setting " 

 the female's secretion in the uterus is similar to that 

 of rennet upon milk.* 'Rennet is milk which contains 

 vital heat, as semen does, and this integrates the 

 homogeneous substance and makes it " set." As the 



' Cf. 755 a 18. This is a remarkable intuition of the 

 essential role played by ferment action in embrjonic develop- 

 ment. Cf. ako Job X. 10 " Hast thou not poured me out as 

 milk, and curdled me like cheese ? Thou hast clothed me 

 with skin and flesh, and knit me together with bones and 

 sinews " (R.V.). 



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