TYPES OF GESTROUS CYCLE 39 



of the previous ovulation. Thus the cow differs from the mouse 

 (see p. 137) where the fohicles which will ovulate at the next 

 oestrous period become distinguishable only during the first half 

 of the preceding dioestrus. 



The cyclic changes in the accessory organs of the cow do not 

 appear to have been entirely worked out. Hammond describes 

 the uterus as secreting a large amount of fluid at oestrus (as 

 occurs in other mammals) and showing congestion during the 

 eight days following heat. According to Murphey the vaginal 

 epithelium undergoes development up to eighteen days after the 

 beginning of the previous heat, and this is followed by general 

 desquamation and reformation of the epithelium during the next 

 week. Haemorrhage from the vaginal stroma, accompanied by 

 leucocytic infiltration, is usual after heat. A more detailed 

 description is given by Frei and Metzger, whose illustrations of 

 the vaginal contents suggest that some prooestrous cornification 

 may occur, as in the rat and mouse. The degree of cornification 

 must be inconsiderable, however, compared with that found in 

 the rodents. 



Hammond has described cyclic changes in the mammary 

 gland of the virgin animal correlated with the oestrous cycle. 

 Before puberty the glands consist merely of ducts, but after the 

 first ovulation there occurs development of lobules of alveoli 

 associated with, but lagging behind, the development of the 

 corpus luteum. The gland never returns to the pre-pubertal 

 state, but regression of the alveoli takes place during the second 

 half of each dioestrus. Hammond does not describe any growth 

 characteristic of the actual period of heat. 



Lactation in the cow does not appear to have any effect upon 

 the recurrence of the cycle in the ovary. Cows are normally put 

 to the bull soon after parturition, and milking is continued 

 until six to eight weeks before the next calf is due. 



{d) HORSE, SHEEP AND PIG 



These animals under conditions of domestication have a 

 dioestrous cycle which probably contains a luteal phase con- 

 trolled by the corpus luteum, but precise experimental infor- 

 mation is lacking. 



