73 6 SCIENCE PROGRESS 



and we must conclude therefore either that it took its origin 

 de novo in that class or that its representative is to be found in 

 lower classes of the Vertebrata. Little is known with certainty 

 concerning the follicular changes following ovulation in the 

 lower vertebrates, for the accounts given by the few workers 

 who have investigated these phenomena do not agree. Wallace 

 (23) describes an enlargement of the epithelial cells accompanied 

 by a more or less marked invasion of the connective tissue and 

 blood vessels in certain fish, although according to some preced- 

 ing workers no such hypertrophy occurs. It is also stated that 

 a somewhat similar series of changes takes place in certain 

 reptiles, and so here perhaps we may have the morphological 

 forerunner of the corpus luteum. 



For the origin of its two main functions described above we 

 shall have to search within the limits of the class Mammalia 

 itself, for both are concerned with processes that are character- 

 istically and exclusively mammalian. Indeed the formation of 

 a placenta is a process that itself originates among the mammals, 

 for it is not found in the lowest sub-class, the Monotremata. 

 These animals (Monotremes) possess corpora lutea, however, 

 which therefore furnish an example of a gland taking on a further 

 and new function within the same class of animals. Moreover, 

 the mammary glands are found in all three sub-classes of the 

 Mammalia and are so characteristic that from their presence the 

 name of the whole class is derived. 



Bibliography 



It is not possible or desirable to give here a full list of the 

 extensive literature of this subject. Certain outstanding papers 

 have been referred to, and these, together with some of the 

 latest investigations on the subject, are given below. In most 

 of these is to be found a list of references to other works. The 

 reader is also referred to The Physiology of Reproduction, by 

 F. H. A. Marshall (London, 1910). 



1. Ancel et Bouin, Sur les Homologies et la Signification des Glandes a Secre- 



tion interne de l'ovaire, Comptes Rend, de la Soc. Biol. t. lxvii. 1909. 



2. , Le developpement de la Glande mammaire pendant la Gestation est 



determine par le Corps jaune, Comptes Rend, de la Soc. Biol. t. lxvii. 1909. 



3. Beard, Rhythm of Reproduction in Mammalia, Anal. Anzeig. Bd. iv. 1897 ; 



The Span of Gestation and the Cause of Birth, Jena, 1897. 



4. Bischoff, Entwickelungsgeschichte des Kanenchenseies, Braunschweig, 1842. 



