J376 THE PHYSIOLOGY OF REPRODUCTION 



placenta. One of the Sirenia, the o\ugong, which possesses a 

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

 presented 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 1 ). The placenta 

 of the mole is not shed at birth, but becomes gradually absorbed 

 by the mother. For such Hubrecht 2 suggested the term 

 c&ntra-deciduate. 



The classification of Strahl 3 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 connec- 

 tion is less intimate, while in the latter haemorrhages occur 

 during pregnancy. But in a physiological sense, the half 

 placenta is certainly as efficient an organ of nutrition as the 

 whole placenta. 



In view of the 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 con- 

 sidered as physiologically 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. Robinson 4 and Assheton 6 

 have recently made efforts in this direction, the former em- 

 phasising the methods of attachment of the trophoblast to the 

 uterus, and the latter the anatomical condition of the tropho- 



1 Turner, "On the Placentation of Halicore Duyong" Trans. Roy. Soc. 

 Edin., voL xxxv., 1889. 



2 Hubrecht, " Spolia Nemoris," Quar. Jour. Micr. Sci., vol. xxxvi., 1894. 



3 See Hertwig, Entivicklungageschichte des Menschen und der Wirbelthiere, 

 1906. 



4 Robinson, " Hunterian Lectures," loc. cit. 



8 Assheton, "The Morphology of the Ungulate Placenta," Phil. Traru. 

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



