TYPES OF (ESTROUS CYCLE 35 



and mouse. This is probably correlated with the fact shown by 

 Loeb that the corpora lutea function in postponing the next 

 oestrus, even in the unmated cycle of the guinea-pig. 



The guinea-pig is similar to the rat and mouse in having an 

 immediate post-partum cestrous period, but it differs from these 

 two animals, according to Loeb and Kuramitsu (406), in that 

 inhibition of oestrus is not found during the rest of lactation. 

 The second post-partum oestrus in the guinea-pig occurs in 

 about sixteen days, i.e. at the normal interval. Lactation, in 

 other words, fails to cause unusual persistence of the corpora 

 lutea from the immediate post-partum ovulation. Since the 

 mouse requires to be suckling more than two young for this 

 result to be produced, the difference in the guinea-pig is probably 

 accounted for by the relatively small demand made on the 

 mother by the smaller-sized, more m.ature litter. 



The ovarian cycle in the guinea-pig is rather remarkable, 

 according to Loeb (398), who has described waves of follicular 

 growth during both dioestrus and pregnancy. Atresia, however, 

 not ovulation, results from this growth. Following ovulation, 

 small follicles become medium-sized in some six days and large 

 in another two, after which atresia sets in. The next wave of 

 follicular growth culminates at the following oestrus and results 

 in ovulation of some of the mature follicles and atresia of the 

 remainder. During pregnancy there are two or more waves of 

 follicular growth ending in atresia. The corpus luteum be- 

 comes fully formed about five days after ovulation. Regressive 

 changes are first visible on the tenth day and accelerate rapidly 

 until the next oestrus. By ten days after the new ovulation, 

 the corpora lutea of the previous oestrus are reduced to small 

 vacuolar bodies surrounded by a connective tissue capsule. 

 During pregnancy the developmental stage of the corpora lutea 

 lasts longer and results in the formation of larger structures. 

 Degenerative changes, similar to those found in the dioestrous 

 corpora lutea, are observed only after the fortieth day. 



The post-ovulation condition of the uterus has been described 

 at length by Loeb. According to his account, there is no marked 

 hypertrophy such as takes place during the luteal phase in the 

 dog, but definite changes occur in the epithelium and stromal 

 glands of the uterus, and in its physiological condition. 



