TYPES OF (ESTROUS CYCLE 43 



ovulation. All mitosis, however, ceases about the end of the 

 first week. At about the tenth day the mucosa cells are again 

 reduced to the low columnar type, with the extrusion of 

 cytoplasmic processes, and the gland epithelium returns to 

 normal. During the last few days of dicestrus the mucosa cells 

 become more elongated and large numbers of leucocytes appear 

 in the stroma. 



This building up of the uterine mucosa during the first half of 

 dioestrus, though comparatively slight, is clearly correlated with 

 the development of the corpus luteum, and is unaffected by the 

 fate of the ova. It may be considered, in the non-pregnant 

 animal, as an abbreviated pseudo-pregnancy. 



Vaginal and mammary changes during the dioestrous cycle in 

 the pig do not seem to have been described. 



As in the cow, mare, and guinea-pig, lactation has no effect on 

 the recurrence of the cycle, except that, according to Marshall 

 (444), there is an interval of five weeks after parturition before 

 the next oestrus. Struve (604), however, states that oestrus 

 recurs four to nine days after parturition. 



(e) MOUSE AND RAT 



In spite of the ease with which the animals can be kept for 

 observation, accurate data on the oestrous cycle of the rat and 

 mouse have only recently become available. Earlier workers 

 were much handicapped by the difficulty of external diagnosis 

 of oestrus, and were compelled to use the immediate post-partum 

 oestrus as their starting-point. Sobotta (583), working from this 

 period, put the length of the cycle in the mouse at about twenty 

 days, while Lataste (358), from the intervals between sterile 

 copulations, placed the length of the cycle at about twelve days. 

 Long and Smith (427), and Smith (572) estimated the length to be 

 sixteen to nineteen days. In the white rat Morau (463), Lataste 

 (358), Long and Quisno (426). and Heape (287), all put the length 

 of cycle at ten days. The discovery, however, that a very defi- 

 nite vaginal cycle exists in both these animals has made it pos- 

 sible to analyse completely the length of the oestrous cycle. In 

 the ordinary unmated mouse and rat oestrus occurs about every 

 five days. Long and Evans (425) for the rat give an average length 



