FCETAL NUTRITION: THE PLACENTA 375 



differences, however, occur, and it is more convenient to de- 

 scribe 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 placentae. 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 classification was introduced by 

 Huxley ^ in 1864. He divided Mammals into two great sections 

 according as their placentae were non-deciduate 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 fcetal villi are simply drawn out 

 Uke the fingers from a glove, no vascular substance from the 

 mother being thrown off. 



In a later pubhcation ^ 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, Amphibia, and 

 Proboscidea ; or discoid, as in Rodentia, Insectivora, Cheiroptera, 

 Lemuridae, Simiadae, and Primates. The Non-deciduata are the 

 Ungulata and Cetacea. The Sirenia and Edentata offer diffi- 

 culties. Of the latter, Manis has a diffuse placenta, Bradypus 

 a poly-cotyledonary, and Orycteropus a discoid and deciduate 



' Huxley, The Elements of Comparative Anatomy, London. 



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

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

 Esohricht, were displaced by those of Huxley. 



' Huxley, Introduction to the Classification of Mammals, London, 1869. 



