66 VERTEBRATE REPRODUCTIVE CYCLES 



Uterine growth already accomplished. Thus progesterone 

 maintains the uterus in a condition suitable for the 

 implantation of the embryo should fertilisation occur. 

 If there is no fertilisation the secretion slackens and 

 finally ceases, but if an embryo implants itself in the 

 uterus the corpus luteum persists and enlarges. 



In pregnancy a second effect of progesterone usually 

 becomes apparent, namely the suppression of any 

 further ovarian activity. This it is able to do because, 

 like testosterone and oestradiol, it has a depressing effect 

 on the secretion of gonadotropic hormone by the anterior 

 pituitary. However this is not the case in all species and 

 it is known that, in the horse and cat,^^^ ovulation may 

 continue during pregnancy. 



In the egg-laying vertebrates bodies which resemble 

 the mammalian corpora lutea, both in method of forma- 

 tion and in structure, have been seen in all classes. The 

 first attempt to ascribe a physiological function to them 

 was made by Bretschneider and de Wit,** who used for 

 this purpose females of a common fresh-water fish, the 

 bitterling. The value of this female as an experimental 

 animal rests in its possession of a long ovipositor, 

 which becomes longer or shorter according to the 

 presence or absence of hormones in the body. It has 

 been found that the most important substance stimulat- 

 ing ovipositor growth is a hormone, which has been 

 called oviductin, and which, though formed in the 

 corpora lutea, is probably not identical with progesterone. 

 However, in this species ovipositor growth must be 

 stimulated before ovulation so that the organ is ready for 

 use when the eggs are shed. Consequently it is not the 

 post-ovulation corpora lutea which are important, but 

 the corpora lutea atretica which result from the break- 

 down of some of the growing oocytes. 



It is considered possible that oviductin may be the key 

 substance which stimulates the final growth of the ovi- 

 ducts of the egg-laying vertebrates, and that in all of 

 them it is produced before ovulation in corpora lutea 



