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other forra, for the trophoblast disappears as a distinct layer, and 

 tlie syncytiuru is formed froui the maternal epithelium. 



The other question, however, of the value of the characters of 

 the placenta in the classification of the remaining Ditremata is 

 one which is not so easy to answer. The classification was origin- 

 ally made at a time when our knowledge of placentation was 

 founded, not on the accurate observation of histological develop- 

 ment, but on such grosser anatomical features as could be displayed 

 by the macroscopical examination of isolated stages, and it is 

 certainly in conflict with other considerations based on the com- 

 parative anatomy of the adults. The difficulties presented by the 

 Edentata for example, (which are stated to include both 'non- 

 deciduate' and 'deciduate' forms) are ignored ; or if it be said 

 that that group is not a natural one then Orycteropus together 

 with Hyrax, Elephas, and Sirenia will be associated with the 

 Carnivora. In the present state of our ignorance these difficulties 

 must necessarily remain ; but it may still be worth while to enquire 

 how far the problem is affected by the recent advances which 

 have been made in the embryology of the groups which we have 

 been particularly discussing. With regard to the foetal membranes 

 and the situation of the placenta in the uterus, it may safely be 

 said that there is for each of these groups a type to which its 

 various members, on the whole, conform. The same is true to a 

 large extent of the minute structure; I may instance the glycogenic 

 layers of the Rodents, the lamellae of the Carnivora, the large 

 trophoblastic sinuses of Mau and Monkeys; and it is significant 

 that in a group like the Insectivora, which is admittedly hetero- 

 geneous, the difïerences are more marked. 



In the Primates we are confronted with a more serious difficulty. 

 As Turner (51) and, more recently, Strahl (48) have shown the 

 placenta of the Lemurs is precisely similar to that of a horse or 

 pig, and difïers totally from that of Monkeys and Man. Balfour, 

 it is true, saw a resemblance between the allantois in Lemurs 

 and Man, and wished on this ground to distinguish the discoidal 

 placenta of the latter from that of Rodents, Insectivores and Bats, 



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