THE OVARY IN MAMMARY-GLAND DEVELOPMENT 



103 



The specific functions of the ovary in parturition probably are more pro- 

 nounced in those forms where it is essential throughout most of the gesta- 

 tional period, such as the viviparous snakes, and among the mammals, such 

 forms as the opossum, rat, mouse, and rabbit. The waning of corpus-luteum 

 activity in these species may serve to lower the level of progesterone in the 

 body and thus permit some of the other factors, such as estrogen or the 

 pituitary principle, to activate the uterus. 



Another factor associated with the ovary and parturition is the hormone 

 relaxin. This substance was first reported by Hisaw and further studied by 

 this investigator and his associates (Hisaw, '25, '29; and Hisaw, et al., '44). 



NON-PREGNANT CYCLE 



PREGNANT CYCLE 



COPULATION NOT COPULATION 



4- PERMITTED J. 



ES TRUS ES T RUS 



PERMITTED 



304 TO 371 



AV 330 TO 345 



Fig. 56. Reproductive and pregnancy cycles in the mare. (Parturition period based 

 upon data supplied by Kenneth ('43); other data supplied by Asdell ('46) and Dukes 

 ('43).) It is to be noted that the first corpus luteum of pregnancy degenerates after 

 about 35 days; the second "crop of corpora lutea" (Asdell) degenerate by 150 days. The 

 ovaries may be removed after 200 days of pregnancy without causing abortion of young. 



Relaxin aids in the production of a relaxed condition of the pelvic girdle, a 

 necessity for the formation of a normal birth passageway for the young. 

 Relaxin somehow is associated in its formation with the presence of proges- 

 terone in the blood stream and also with the intact reproductive system. 

 Relaxin together with estrogen and progesterone establishes a relaxed con- 

 dition of the tissues in the pubic area of the pelvic girdle. 



I. Importance of the Ovary in Mammary-Gland Development 

 and Lactation 



Estrogen and progesterone together with the lactogenic hormone, luteo- 

 trophin, of the pituitary gland are necessary in mammary-gland development. 

 The entire story of the relationship of these and of other factors in all mam- 

 mals or in any particular mammal is not known. However, according to one 

 theory of mammary-gland development and function, the suggestive roles 

 played by these hormones presumably are as follows (fig. 58): Estradiol and 

 other estrogens bring about the development of the mammary-gland ducts; 

 as a result a tree-like branching of the ducts is effected from a simple im- 



