114 FOLLICULAR DERIVATIVES IN VERTEBRATES III 



fishes (e.g., Zoarces), whose calyx nutricius may show a 

 weakly luteinised granulosa. 



In reptiles we see for the first time, phylogenetically, a 

 genuine post-ovulation corpus luteum. We can distinguish 

 two groups: (a) the oviparous reptiles, with a short-lived, 

 incompletely organized post-ovulation corpus luteum, and 

 (b) the ovoviviparous and viviparous reptiles, with a long- 

 lived, completely organized post-ovulation corpus luteum. 



In birds there is added to this double provision of corpora 

 lutea, as an inheritance from their Sauropsidan-organization, 

 the atretic element. As in oviparous reptiles, the post- 

 ovulation corpus luteum shows only incomplete organization. 

 At the same time there is abundant formation of corpora 

 atretica, which, in the case of follicles with large ova rich 

 in yolk, frequently become ruptured, and give rise to a large 

 corpus atreticum ruptus. 



In Menotremata there is no pre-ovulation corpus luteum, 

 but a well-formed post-ovulation corpus luteum, accom- 

 panied by numerous large corpora atretica. There are two 

 phenomena which characterize Monotremata: (a) numerous 

 large corpora atretica rupta — probably a result of oviparity 

 and the fact that their ova are rich in yolk as in Sauropsid- 

 ans, and (b) the well-formed post-ovulation corpus luteum 

 for the provision of nourishment for the young, pointing to 

 the condition in placental mammals. 



In most placental mammals the post-ovulation corpus 

 luteum predominates; it is, however, always accompanied by 

 different forms of atretic follicular derivatives. During the 

 last decade some exceptions of more theoretical importance 

 have come to light; for example, the Elephantulus type, in 

 which ovulation is preceded by the organization of the 

 corpus luteum, and its hormonal function. Closely akin to 

 this form of retention of the ovum is a second iorm of 

 retention of the pre-ovulation corpus luteum, which occurs 

 regularly in addition to the ordinary post-ovulation corpus 

 luteum in the rat (Long and MCLEAN, 1922); the marc 

 (Cole, Howell and Hart 193 i ) ; the porcupine (Erethion 

 dorsatus, MOSSMAN, 1940) and Macaca rhesus (CORNER, 



