246 



THE HISTORY OF CEEATION. 



maternal uterus. For in the three lower orders of Placental 

 animals, in Hoofed animals, Whales, and Toothless animals, 

 the peculiar spongy membrane, which is called the deciduous 

 riieTnhrane, or decidua, and which connects the maternal and 

 the foetal portions of the placenta, does not become de- 

 veloped. This takes place exclusively in the seven higher 

 orders of Placental animals, and we may, therefore, according 



to Huxley, class them in the main group of Dedduata, or 

 animals with decidua. They are contrasted with the three 

 first-mentioned legions of indeciduous animals, or Inde- 

 ciduata. 



But in the various orders of Placental animals the placenta 

 differs not only in important internal differences of struc- 

 ture, which are connected with the absence or the presence 



