198 EMBRYOLOGY 



POSTERIOR CARDINAL 



CAPILLARY NETWORK 



UREA 

 MESONEPHRIC DUCT 



SUBCARDIAL 



Fig. 118. A schematic representation of a functional unit of the mesonephros. 

 The circulation ,within the mesonephros is not as simple as shown. Blood from 

 the posterior cardinal vein flows through ill-defined sinuses between the tubules 

 and is collected by a subcardinal vein. 



the metanephros begins in late development and continues in the adult. It 

 excretes chiefly uric acid, a substance which is relatively insoluble and thus 

 can be stored without taking up large amounts of water. Since the embryo 

 develops within a sealed cavity, the shell, it is absolutely necessary that its 

 excretions be in a concentrated form. The properties of uric acid are much 

 more suitable for this purpose than those of urea, which takes up large 

 amounts of water. Thus in the embryo of the bird the process of excretion 

 becomes adapted to an existence which requires a minimum of volume of 

 excretory products. It is interesting to point out that this same adaptation is 

 very useful to the adult bird, for it reduces the weight for flying. 



The uric acid of the embryo accumulates in the cavity of the allantois 

 during development. After hatching, the dried up allantois and the excretions 

 are left within the shell. 



Parts of the degenerating mesonephros are utilized by the developing 

 reproductive system in the male. Some of the tissue of the mesonephros 

 becomes converted into the epididymis, while the Wolffian duct forms the 

 vas deferens, which carries the sperm from the testis to the cloaca. In the 



