FCETAL NUTRITION: THE PLACENTA 409 



Edentata offer difficulties. Of the latter, Manis has a diffuse 

 placenta, Bradypus a poly-cotyledonary, and Orycteropus a zonary and 

 deciduate placenta. One of the Sirenia, the dugong, which possesses 

 a zonary but not deciduate placenta, illustrates a type not repre- 

 sented at all in Huxley's classification. No maternal tissue is lost at 

 birth ; but, in addition, part of the foetal tissue remains attached to 

 the uterus and is absorbed (Turner i). The placenta of the mole 

 is not shed at birth, but becomes gradually absorbed by the mother. 

 For sucb Hubrecht ^ suggested the term contra-deciduate. 



The classification of Strahl ^ does not promise to be any more 

 satisfactory. He divides Mammals into two groups, one having a 

 " Halbplacenta " and the other a " Vollplacenta." In the former no 

 maternal vessels are opened and the connectioii is less intimate, while 

 in the latter htemorrhages occur during pregnancy. But in a 

 physiological sense the half placenta is certainly as efficient an 

 organ of nutrition as the ivhole placenta. 



In view of the more recent work on the placenta, it is obvious 

 that Huxley's classification fails in taking no account of the 

 trophoblast, the most active constituent of the placenta, and in laying 

 too much stress on the differences at birth, i.e. on the shedding of an 

 organ which is of no more use, and may be considered as physio- 

 logically dead. Moreover, it would appear that in many of the 

 deciduate Mammals almost no maternal tissue except blood is lost at 

 birth, and maternal blood is also lost in the non-deciduate sheep. A 

 perfect classification must take account of the structure and behaviour 

 of the trophoblast during the whole course, or at least the earlier 

 part, of pregnancy. Without it a clear insight into the processes 

 which regulate foetal nutrition cannot be obtained. Eobinson * and 

 Assheton ^ have made efforts in this direction, the former emphasising 

 the methods of attachment of the trophoblast to the uterus, and the 

 latter the anatomical condition of the trophoblast at the time of its 

 first attachment. Hubrecht, on the basis of Huxley's statement that 

 Insectivora are among the most archaic of Mammals, has investigated 

 several members of this order as showing probably the most ancient 

 type of placenta, and thus affording a starting-point for a classifica- 

 tion. According to Huxley, the least differentiated types, the 

 hedgehogs and Gymnura, occupy a central position, while shrews 

 show resemblances to Eodents, and Twpaioi to lemurs; moles and 

 Galeopitheci vary m other directions, while the whole order shows 



1 Turner, " On the Placentation of Halicore dugong," Trans. Roy. Sac. Edin., 

 vol., XXXV., 1889. ' 



2. Hubrecht, "Spolia Nemoris," <^uar. Joiir. Micr. Science, vol. xxxvi., 1894. 



' See Hertwig, Entwicklungsgeschickte des Mensc/ien ivixdder Wirhelthiere, 1906. 



^ Robinson, Hunterian Lectures, loc. cit. 



■> Assheton, " The Morphology of the Ungulate Placenta,'' Phil. Trans. Roy. 

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



