ORGANS OF INTERNAL SECRETION 339 



is at its height, the corpus luteum is often reduced to httle more 

 than a scar. Fraenkel also lays some stress on the discovery 

 that the luteal cells are derived from the folUcular epithehum 

 and not from the connective tissue of the stroma. Furthermore, 

 he observes that whereas many cases have been recorded in 

 which double ovariotomy was performed during pregnancy 

 without interfering with the further course of development, in 

 none of these, so far as he is aware, was the operation conducted 

 in the early weeks. 



Fraenkel observes also that in non-placental Mammals 

 (Marsupials and Monotremes) the corpus luteum is rudimentary 

 or does not exist at all. Sandes,^ who has carefully described 

 the formation of the corpus luteum in the marsupial cat, points 

 out that this is erroneous, and says that there is a large corpus 

 luteum in the members of both these groups. It should be 

 remembered, however, that in Marsupials the embryo is 

 nourished by a " yolk-sac placenta," while in at least one genus 

 (Perameles) a definite allantoic placenta exists. In Monotremes 

 there is a pronounced hypertrophy of the follicular epithehum 

 following upon ovulation, but the corpus luteum is not normal 

 in this group, since there appears to be no ingrowth of con- 

 nective tissue or blood-vessels from the follicular wall (see p. 149). 



A similar objection, that might be raised in opposition to 

 Fraenkel's hypothesis, is that structures resembhng corpora 

 lutea have been found in the ovaries of certain of the lower 

 Vertebrates (see p. 151). This resemblance relates chiefly to the 

 hypertrophy of the cells of the follicular epithehum after the 

 discharge of the ova. Such an objection is not to be regarded 

 as a serious one, for there is nothing improbable in the sup- 

 position that rudimentary corpora lutea, providing probably 

 some sort of secretion, should have been developed before the 

 acquirement of the function, which, according to Fraenkel's 

 hypothesis, is possessed by the fully formed structure which 

 characterises the placental Mammaha. 



Fraenkel has also pointed out, as an argument in favour of 



his theory, that in ectopic or extra-uterine pregnancy the uterus 



undergoes the usual changes although there is no ovum in 



the uterine cavity. It is clear, therefore, that the changes do 



' Sandes, he. cit. 



