406 Growth of the Toung in the Egg. [Book IX, 



the internal furface of the uterus, and furrounded by 

 very thin membranes. It feems formed, however, of 

 two mafles joined together, the larger of which is the 

 head and the fmaller the body. As the foetus' advances 

 in growth the body acquires a larger fize with refpect 

 to the head, fmall protuberances make their appearance 

 on the body, which are the future limbs, and the fea- 

 tures begin to manifeft themfelves; In this manner 

 the foetus, gradually acquiring a more determinate 

 ftrubure, and more evident marks of the fpecies to 

 which it belongs, is at length difengaged from the 

 mother. In different fpecies there is great variety in 

 the perfection of the animal at the time of birth ; the 

 young of the human fpecies is, perhaps, the mod back- 

 ward of any in this refpecl: -, for' a- child, when fix 

 months old, is not fo able to provide for itfelf as a 

 horfe or an afs at the age of as many days. 



In many races of animals it Ihould be obferved, 

 particularly in birds, the growth of the fcetus takes , 

 place out of the body of the mother. This is indeed 

 the cafe with all animals which fpring from eggs, and 

 in which we have a very favourable opportunity of ob- 

 ferving the progrefs of the fcetus from its firft appear- 

 ance till it has acquired that ftate of perfection at which 

 it is hatched. During the whole period of its growth 

 it is fupported by a limited quantity of nourifhmenc 

 contained within the egg- (hell, and which is that part 

 of the egg called the yolk. 



The human fcetus is furrounded with three mem- 

 branes; the external of thefe is vafcular, and is called 

 the fpongy chorion ; the middle coat, called the media 

 or true chorion, and the internal one, called the ana- 

 pion, are not fo. Mr. Hunter has found the fpongy 

 chorion to confift of two, layers j that which lines the 

 uterus he calls membrana caduca or dssidua^ becaufe it 



