440 OUTLINES OF CHORDATE DEVELOPMENT 



I. Mammalia ovipara. Monotremata. 

 II. Mammalia vivipara. 



A. Achoria (Aplacentalia) . Most Marsupialia (the exceptions 

 are Perameles, Dasyurus, and perhaps Phascolarctos) . 



B. Choriata (Placentalia) . 



1. Semiplacentalia (Partial placenta). Chorionic and 

 uterine structures in close apposition but not fused; 

 simple separation at parturition. 



a. Semiplacenta avillosa. Chorion without villi. Pera- 

 meles, Dasyurus. 



b. Semiplacenta diffusa. Simple villi uniformly dis- 

 tributed. Horse, pig, tapir, hippopotamus, camel, 

 deer, whale, Manis, Lemurs. 



c. Semiplacenta multiplex. Villi in groups or cotyle- 

 dons. (Cotyledonary placenta.) Ruminants. 



d. Semiplacenta zonaria. Villi in a zone or band 

 around the chorion. Only in the Dugong (Halicore) . 



2. Placentalia vera (Complete placenta). More or less 

 complete fusion of chorionic and uterine tissues, in- 

 volving tissue destruction at parturition. 



e. Placenta zonaria. Placental fusion in the form of a 

 broad band or zone, completely around the chorion. 

 Most Carnivora. In Hyrax and the elephant com- 

 bined with cotyledons. 



f. Placenta zono-discoidalis. Placental fusion in the 

 form of an incomplete band or zone. Mustelidce 

 (e.g., marten, weasel, ferret, etc.). 



g. Placenta discoidalis. Placental fusion in the form 

 of a circular disc, usually simple in outline, though 

 sometimes lobed. Insectivors, bats, Rodents, Tar- 

 sius, apes, man. 



Of the many other classifications* of placental types we may 

 mention only one, that of Grosser, based upon an entirely 

 different relation, namely, the extent of the erosion of the 

 maternal uterine tissues effected by the trophodermal cells of 

 the chorion. From this point of view placentae are arranged 

 in four groups, as follows: 



I. Placenta epitheliochorialis. All maternal tissues retained, un- 

 eroded. Chorionic epithelium (of the villi) in contact with the 

 uterine epithelium. Pig. 



