XIIl] FEMALE GENERATIVE ORGANS 183 



however, are usually s^^ollen and mucus is emitted. The clitoris 

 and vulva often undergo a succession of spasmodic movements, 

 Ijreceded by the discharge of small quantities of urine and this 

 fluid may have the consistenc}' of oil. Suckling mares tend to 

 fail in their milk suj)i3ly, and the quality of the milk appears to 

 undergo some kind of change, as it is frequently found that foals 

 during the heat periods of their dams suffer from relaxation of the 

 bowels or even acute diarrhoea. In mares which are not suckling 

 the mammary gland becomes congested and increases in size during 

 the heat. At the same time some mares develop great excitability, 

 and kick and squeal, becoming dangerous to approach and im- 

 possible to drive. There is, hoAA-ever, great variation, for other 

 animals may pass through the 'heat' period without exhibiting 

 any well-marked signs of their condition which in a few instances 

 can only be determined b}^ the behaviour of the mare towards the 

 stallion. 



Of other domestic animals the cat is polyoestrous and has a 

 dioestrous cycle of about 14 days and three sexual seasons annu- 

 ally, and the period of gestation is nine weeks. The ferret is 

 monoestrous and has two or three sexual seasons which are however 

 usually confined to spring and summer. Gestation lasts 40 days. 

 The rabbit and other wild rodents are polj'oestrous. 



In the mare, cow, ewe, sow and bitch ovulation takes j^lace 

 spontaneously, but in the cat, rabbit and ferret it requires the addi- 

 tional stimulus set up by coitus before it can occur. Presumably 

 coitus causes a nerve reflex. In the rabbit ovulation takes place 

 from 9| to 10 hours after coitus. Maturation of the ovum also 

 depends upon the occurrence of coitus and commences shortly 

 afterwards, but is not completed until after ovulation. In such 

 animals as the rabbit which do not ovulate spontaneously, corpora 

 lutea are not usually formed apart from pregnancy, and the 

 pseudo-pregnant condition does not normally occur. 



Pregnancy. The ova are fertilised by the spermatozoa in the 

 Fallopian tube. A spermatozoon after coming in contact with an 

 ovum passes bodily through its wall, the head or nucleus of the 

 spermatozoon fuses with the nucleus of the ovum thus restoring 

 the amount of nuclear material or number of chromosomes to 

 that characteristic of the species, while the tail of the sjiermato- 

 zoon breaks up and mingles with the external (or non-nuclear) 

 proto])lasm of the ovum. The fertilised ova then pass down the 



