MAMMALIAN SEXUAL CHARACTERS 143 



it must be remembered that the ovum of Monotremes 

 is known to remain for a short period in the oviduct, 

 or in other words to pass through it very slowly, and 

 to absorb fluid from its walls, as shown by the 

 considerable increase in size which the ovarian 

 ovum undergoes before it is laid. It would be in- 

 teresting to know how long the rudimentary corpus 

 luteum persists in Ornithorhynchns : the period, 

 according to my views, should be very short. It 

 is remarkable that in the results quoted by Marshall 

 a well-developed corpus luteum was found and 

 exclusively found in the lower Vertebrates which are 

 viviparous. For example, among fishes in the Elas- 

 mobranchs Myliohatis and Spinax ; in Teleosteans, 

 in Zoarces ; in Reptiles, in Anguis and Seps. Biihler, 

 on the other hand, confirmed my own negative 

 result with regard to oviparous Teleosteans, and also 

 found no hypertrophy of the follicle in Cyclostomes 

 which are also oviparous. In the viviparous forms 

 mentioned there is yolk in the ovum which is re- 

 tained in oviduct or ovary, but additional nutriment 

 is also absorbed from the uterine or ovarian walls. 

 In these cases there is no placenta and generally no 

 adhesion of ovum or embryo to walls of oviduct or 

 ovary. These facts alone would be sufficient to 

 disprove the theory that the corpora lutea are organs 

 producing a secretion whose function is to cause the 

 attachment of the embryo to the uterine mucosa. 

 It is also, in my opinion, unreasonable to sujopose 

 that the rudimentary corpora lutea of lower vivi- 

 parous Vertebrates arose as a mutation the result 

 of which was to cause internal development of 

 the ovum. Habits might easily bring about reten- 

 tion of the fertilised ova for gradually increasing 



