STRUCTURE OF THE VERTEBRATES 95 



denary placenta, consisting of rounded masses of villi spaced 

 at intervals over the chorion, develops in the cow, sheep and 

 other herbivorous animals; (4) an intermediate type, combin- 

 ing types (2) and [S) is present in a few herbivores; and (5) 

 a zonary placenta develops in the carnivores, consisting of a 

 median band around the chorion. 



With its efficient feeding mechanism the placental mammal 

 is born in a fairly well developed condition. The degree of de- 

 velopment varies with different orders and even the genera 

 within an order. In general terms, the more primitive mammals 

 are born in a less mature condition, but even there one must dis- 

 tinguish between total development and the specialization of 

 certain characters. The newborn rat, for example, is able to 

 crawl although its eyes are not open; a newborn monkey has its 

 eyes open but has very limited body coordinations. The length 

 of the embryonic period is fairly correlated with size at birth, 

 although the body size of the parents is a contributing factor. 

 Further, the length of this period of gestation and the rapidity 

 of post-natal development are correlated with the total life 

 span of the race. The poor development of the human at birth, 

 and the prolonged period of development have been variously 

 interpreted. 



