ROLE OF THE OVARY IN PARTURITION 101 



Tiger (Felis tigris) 106 



Whale (Various species) 334-365 



Wolf (Canis lupus) 63 



Woodchuck (Marmota monax) 35-42 



Zebra, mountain (Equus zebra) 300-345 



3. Maintenance of Pregnancy in Reptiles and Other 

 Vertebrates 



In certain viviparous species of the genera Storeria, Matrix and Thamnophis, 

 Clausen ('40) reports that ovariectomy during gestation results in resorption 

 of the embryo when performed during the earlier phases of gestation and 

 abortion during the middle of gestation, but during the terminal portion of 

 pregnancy the process is unaffected and the young are born normally. These 

 results are similar to those obtained from the rabbit as noted previously. 



While experimental evidence is lacking in other vertebrate groups which 

 give birth to the young alive, the evidence obtained from reptilian and mam- 

 malian studies suggests that hormones are responsible for the maintenance 

 of pregnancy. In harmony with this statement, it may be pointed out that 

 in the viviparous elasmobranch fishes (e.g., sharks) corpora lutea are de- 

 veloped in the ovaries. 



H. Role of the Ovary in Parturition or Birth of the Young 



The real factors bringing about parturition are not known, and any ex- 

 planation of the matter largely is theoretical. However, certain aspects of 

 the subject have been explored. For example, it was observed above that 

 progesterone appears to antagonize the action of estrogen with the result 

 that the uterus stimulated to irritability and contractility under the influence 

 of estrogen is made placid by the action of progesterone. In harmony with 

 this action studies have shown that estrogen tends to increase during the 

 final stages of normal gestation, while progesterone appears to decrease, ac- 

 companied by an involution of the corpora lutea. Consequently, the foregoing 

 facts have suggested the "estrogen theory," which postulates that activities 

 of the uterine musculature are increased by the added amounts of estrogen 

 in the presence of decreasing amounts of progesterone during the latter phases 

 of pregnancy. In confirmation of this theory, it has been shown that proges- 

 terone injected into a pregnant rabbit near the end of the gestation period 

 will tend to prolong gestation. A second theory of parturitional behavior 

 assumes that the posterior lobe of the pituitary gland elaborates oxytocin 

 which induces increased uterine activity, resulting in birth contractions 

 (Waring and Landgrebe in Pincus and Thimann, '50). Again, a third concept 

 emphasizes the possibility that the placenta may produce substances which 

 bring about contractions necessary for the expulsion of the young (Turner, 

 '48, p. 428). Oxytocic substances have been extracted from the placenta, 

 which suggests the validity of this theory. 



