REPRODUCTION 8i 



of ovulation or pseudopregnancy; for the next few days their development 

 parallels that of corpora of pseudopregnancy. On the eighth day, however 

 (in the mouse, 38), they begin a period of rapid growth largely accomplished 

 by an increase in cell size, and by the sixteenth day they are almost twice 

 the diameter of corpora of ovulation. They are made more conspicuous 

 by the fact that, during pregnancy, ovulation is suspended and no new 

 corpora formed, while meantime older sets of corpora rapidly regress. 

 With appropriate vital stains traces of the corpora of pregnancy may be 

 detected for three or four months post partum in rat ovaries. 



4. Corpora lutea of lactation are the corpora that develop in non-pregnant 

 nursing mice from the follicles that ovulate at the first post partum estrus. 

 These corpora are distinguished from all others by the particularly small 

 size of the lipoid granules. Within twenty-four hours of the removal of a 

 nursing litter, the granules show the characteristic increase in size indicative 

 of cessation of function of the corpora. Corpora lutea of lactation attain a 

 size somewhat larger than that of corpora lutea of ovulation or pseudo- 

 pregnancy, but not equal to that of corpora lutea of pregnancy. 



Lactation. — The normal duration of lactation in mice is about four 

 weeks. Milk production rises for the first ten days and then gradually 

 declines (46). In lactating mice a long period of diestrus occurs following 

 the first post partum estrus. If the nursing litter is of normal size, this 

 diestrous period, called the lactation interval, lasts from about 20 to 25 days 

 (34, T,'^). It may be terminated by removal of the litter, this inducing 

 estrus in two to four days. If the stimulus of suckling is maintained by 

 replacing the growing litters from time to time by very young litters, an 

 estrus occurs as usual at about three weeks, but future estrous periods are 

 delayed, the intervening diestrous intervals being some twelve to seventeen 

 days long (116). The stimulus of nursing can produce a marked effect on 

 females who have not recently lactated. Thus when normally cyclic 

 adult mice or rats are given on alternate days new litters of actively nursing 

 young, marked development of the mammary glands occurs. Milk secre- 

 tion may even be induced. The condition is accompanied by a lengthening 

 of the diestrous interval to two or three weeks, and by the appearance of the 

 capacity for deciduomata formation in the uterus. When the nursing 

 stimulus is removed, normal estrous cycles commence within a few days 

 (116, 117, 118). It may be noted in this connection that deciduomata 

 formation may also be induced during normal lactation (83, 85). Mammary 

 involution following removal of a litter may be retarded in mice by irritation 

 of the nipples with turpentine applied twice daily (68). 



