THE EARLY DEVELOPMENT OF THE MAMMAL 439 



essential facts of placental arrangement. The earlier classi- 

 fications, such as those of Owen, Huxley and Kolliker, based 

 upon the type of villous distribution and the degree of intimacy 

 between the villi and the uterine mucous membrane, may be 

 summarized as follows, following Hertwig in the main : 



I. Achoria. Chorion with few or no villi. Monotremes and Marsupials. 

 II. Choriata. Chorion with many villi. 



A. With uniformly distributed villi, not intimately related 

 with the maternal tissues. Most Ungulates except the 

 Ruminants (e.g., horse, pig, camel, deer, etc.). 



B. With villi localized in definite areas, and more or less closely 

 related with maternal tissues. The true Placentalia. 



1. Villi in numerous small patches or cotyledons. Cotyle- 

 donary placenta. Ruminants. 



2. Villi in a band or zone around the chorion. Zonary 

 placenta. Carnivora. 



3. Villi in a single laige circular area. Discoid placenta. 

 Insectivors, Bats, Rodents, higher Primates including 

 man. 



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The terms deciduate and non-deciduate in the classification 

 above require a word of explanation. In several groups of 

 Mammals the epithelium of the mucous membrane lining the 

 uterine cavity becomes greatly thickened during pregnancy, 

 or preceding menstruation. This thickening during pregnancy 

 is termed the decidua, and usually the relations of the villi to 

 the decidua are such that the separation of the placenta at 

 parturition involves a certain loss of maternal tissue (de- 

 ciduata). In other forms, however, the uterine epithelium 

 shows no such proliferation, and the chorionic villi, at parturi- 

 tion are simply withdrawn from the pits in the mucosa where 

 they have been lodged, and no destruction of maternal tissues 

 results (non-deciduata). An intermediate condition, known as 

 contra-deciduate, is found in the mole where, although a decidua 

 is formed, the placental tissues are not lost at parturition, but 

 are absorbed by the walls of the uterus. 



A more recent and more detailed classification of placental 

 arrangement, again based upon the morphological arrangement 

 of the villi, is that of Strahl, as follows: 



