408 THE PHYSIOLOGY OF REPRODUCTION 



in the uterus itself. In the sheep, Asshetori, 1 detected no difference 

 except an increase in the number of the leucocytes. There was no 

 sign of activity in the uterine glands or blood-vessels. When the 

 ovum reaches the uterus changes begin dilatation of blood-vessels 

 and lymphatics, widening and increased tortuosity of glands, 

 disappearance of cilia from the surface epithelium. The whole 

 mucosa is soft and oedematous, and there may even be a transudation 

 of lymph entering the uterine cavity and mingling with the glandular 

 secretion to form a supply of nutriment for the ovum before attach- 

 ment. Great differences, however, occur, and it is more convenient 

 to describe the changes in the uterine mucosa in each order. 



IV. PLACENTAL CLASSIFICATION 



At the outset we are beset with the difficulty of grouping 

 Mammals in such a way as to show how the variations in the 

 anatomy and physiology of the placenta have been evolved. Well- 

 marked differences, such as occur in other organs and serve to 

 differentiate Mammals into certain orders, are not always to be 

 observed in their placenta. In widely diverging groups there may 

 be striking similarities in placentation, while great differences may 

 exist in closely related types. On this account the most satisfactory, 

 and indeed the only possible, classification of Mammals for our 

 purpose is one based on their placental characters. Such a classifica- 

 tion was introduced by Huxley 2 in 1864. He divided Mammals into 

 two great sections according as their placentae were non-decidnate or 

 deciduate? In Deciduates the substance of the mucosa undergoes 

 rapid growth and textural modification to form decidual tissue, and 

 the maternal and foetal parts of the placenta become firmly united. 

 In Non-deciduates there is no formation of decidual tissue, and at 

 parturition the foetal villi are simply drawn out like the fingers from 

 a glove, no vascular substance from the mother being thrown off. 



In a later publication 4 Huxley attempted to arrange all Mammals 

 in one or other division. The Deciduata are classed in two groups 

 according to the external appearance of the placenta, which is either 

 zonary? as in Carnivora and Proboscidea ; or discoid, as in Rodentia, 

 Insectivora, Cheiroptera, Lemuridae, Simiidse, and Primates. The 

 Non-deciduata are the Ungulata and Cetacea. The Sirenia and 



1 Assheton, " The Morphology of the Ungulate Placenta, etc.," Phil. Trans. 

 Roy. Soc., London, Ser. B., vol. cxcviii., 1906. 



2 Huxley, The Elements of Comparative Anatomy, London. 



3 Thirty years earlier Weber had suggested a similar division into caducous 

 and non-caducous ; but his terms, although accepted by von Baer and Eschricht, 

 were displaced by those of Huxley. 



4 Huxley, Introduction to the Classification oj Jiammals, London, 1869. 



6 When the chorionic villi are limited to an annular area the placenta is 

 called zonary. 



