§ 6 DERIVATIVES AND INTERSTITIUM 109 



Frequently they still show cyclic changes by growing smaller 

 in the interestrus, expanding again in the estrus, and behaving 

 as glandular tissue. In Selachii, Bretschneider and Van 

 Egmond observed a persistance of all granulosa-lutein cells 

 of the pre-ovulation corpus luteum as large lamellae, con- 

 stituting the major part of the large interstitium. The cells 

 retain their glandular character and show cyclic changes as 

 long as the gland is well supplied with blood. 



To summarize, we may say that there exists a morpho- 

 logical, and probably also functional, contrast between cor- 

 pora atretica and corpora lutea. The atresia produces a theca 

 gland; the corpus luteum forms a granulosa gland. In so far 

 as mixed forms occur, theca-luteinisation predominates in 

 the corpus atreticum, and granulosa-luteinisation in the 

 corpus luteum, both as regards the tissue and as regards 

 hormone production. The corpus atreticum mixtum, more- 

 over, occurs only rarely; if at all, only in young follicles. 

 The pre- and post-ovulation corpora lutea, on the contrary, 

 show, morphologically, an almost continuous series, so that 

 in borderline cases, such as the pre-ovulation corpus luteum 

 with retention of the ovum, or the Elephantulus type, we 

 may frequently be in doubt as to the group with which it 

 should be classed. 



A different question altogether is which hormone is form- 

 ed by the pre-ovulation corpus luteum, and which by the 

 post-ovulation corpus luteum. For an answer to this question, 

 adequate data are lacking in many cases. We know that the 

 pre-ovulation corpus luteum in Rhodeus and Lophius forms 

 oviductin, and that the post-ovulation corpus luteum in 

 mammals produces progesterone. These two sexual hormones 

 are not identical, as was already stated in the Introduction. 

 The products of the other pre- and post-ovulation corpora 

 lutea, of amphibia, reptiles and birds, or of the atretic fol- 

 licular derivatives, are unknown. Oviductin and progesterone 

 represent, after all, the two extremes; and the question of 

 where, in the range of vertebrates, oviductin stops and pro- 

 gesterone begins is of some theoretical, and, maybe, also of 

 practical importance. 



