536 THE DEVELOPMENT OF THE INDIVIDUAL 



usually bring about secondary sex changes which will stimulate the 

 growth of the appropriate organs and also restore the correct physio- 

 logical conditions for that sex. 



The Embryonic Membranes. Three of these membranes first appear 

 in the reptiles and play a vital part in the life of the embryo within the 

 shell. The most primitive of the extraembryonic membranes is the 

 yolk sac which appears in the fish embryos. This is a living membrane, 

 containing a network of blood vessels which carry food from the egg 

 to the developing embryo by means of the vitelline arteries and veins. 

 Curiously, this sac develops in mammals as an empty cavity, since the 

 tiny egg of this class contains little or no yolk. A vestige of this sac 

 may persist in adults as a tiny projection on the small intestine. 



The second membrane, the amnion, is formed around the embryo 

 of land vertebrates and becomes filled with amniotic fluid in which the 

 embryo floats. Thus land vertebrates have created an aquatic environ- 

 ment for their offspring which supports and protects the delicate parts 

 of the embryo during development. This gives excellent protection from 

 pressure or blows, since according to a law of physics, force is dis- 

 tributed equally in all directions by a liquid. 



The third membrane, the allantois, is formed as an outpocketing of 

 the hindgut of the embryo anterior to the proctodeum. In reptiles and 

 birds, this membrane grows around the amnion and yolk sac in the space 

 formed by the chorion, where it acts as an embryonic lung giving off 

 carbon dioxide through the shell and absorbing oxygen. In mammals 

 the allantois forms the placenta by coming into intimate contact with the 

 maternal uterus. The allantoic blood vessels become the umbilical 

 arteries and veins which reach the growing embryo through the long 

 umbilical cord. In human beings the endodermal comoonent of the 



o i 



allantois is poorly developed, but the blood vessels are still the all-im- 

 portant link between the embryo and the mother. The blood of the 

 mother does not enter the embryo, but food and oxygen diffuse through 

 the membranes of the placenta into the blood vessels and the wastes pass 

 out the same way. 



A fourth membrane, the chorion, is a very thin, unimportant mem- 

 brane that lies next to the shell in birds and reptiles. It is formed more 

 or less by accident from the outer layer of the amniotic folds when they 

 come together and fuse over the embryo. In human beings the chorion 

 is more important since, in the very young stage, it is the layer which 

 bores its way into the uterus and absorbs the first nourishment before 

 the allantoic blood vessels develop. Thus, the placenta is formed in 

 part from the chorion as well as from the allantois. 



