DIFFERENTIATION OF SOMITES 507 



In fact, by the fifth day, circulation has already been established in the 

 glomeruli, and from then until the eleventh day, the mesonephros is at 

 the height of its functional activity. Then the metanephros takes its 

 place. 



The pronephric tubules which attain even a degree of completeness 

 lie in the tenth to fifteenth somites. It is interesting to observe that 

 when these tubules begin to degenerate, the glomeruli begin to form at 

 the points of the tubules, close to the coelom and actually project into 

 the coelom. These bud-like structures are extremely variable, both as to 

 number and degree of development. They even develop differently on 

 both sides of the chick. They appear to be best developed on the third 

 and fourth days. It is for reasons such as the ones mentioned above 

 that former writers insisted that in the chick the pronephros really de- 

 veloped later than the mesonephros. 



