1 62 THE EVOLUTION OF MAN. 



some degree connected with other important structural 

 characters (e.g., the structure of the brain, of the teeth, of the 

 feet), and which may justly, therefore, be turned to account 

 in the phylogenetic classification of Placentals. In the first 

 place, two great groups of Deciduata may be distinguished 

 according to the form of the placenta: in the one group it 

 is ring-shaped or girdle-shaped; in the other it is discoid or 

 cake-shaped. In Deciduata with girdle-shaped placenta 

 (Zonoplacentalia) the poles of the oval egg take no part 

 in the formation of the placenta. The " vascular cake " 

 resembles a broad ring-like girdle, embracing the central 

 zone of the egg. It is so in Beasts of Prey (Camassia), 

 both in the terrestrial forms (Carnivora) and in the marine 

 forms (Pinnipedia). A similar girdle-shaped placenta is 

 found in the False-hoofed Animals (Chelophora) : the 

 elephants, and Klip Das (Hyrax) with its allies, which were 

 formerly classed as Hoofed Animals. All these Zonoplacen- 

 talia belong to one or more side-branches of the Deciduata, 

 which are not nearly allied to Man. 



The second and most highly developed group is formed 

 by the Deciduata with discoidal placenta (Discoplacentalia). 

 The formation of the placenta is here most localized and 

 its structure most fully developed. The placenta forms a 

 thick, spongy cake, usually in the form of a circular or 

 oval disc, and attached only to one side of the uterine 

 wall. The greater part of the embryonic egg-membrane is, 

 therefore, smooth, without developed tufts. To the Disco- 

 placentalia belong the Semi-apes and Insect-eaters, the 

 Diggers (Effodienta) and the Sloths, Rodents and Bats, 

 Apes and Man. Comparative Anatomy enables us to infer 

 that of these various orders the Semi-apes are the parent- 



