STRUCTURE OF FOUR-DAY CHICKS 



149 



fluid passing through the tubule is drawn from the glomerulus 

 and other capillaries adjacent to the tubule. 



In the chick a few of the more anterior mesonephric tubules 

 are of the primitive type and show vestiges of a nephrostome 

 opening into the coelom (Fig. 52, C). These anterior meso- 

 nephric tubules, however, persist for but a short time, do not 

 attain the characteristic relation to a glomerulus and never 

 become functional. Even in chicks of four days' incubation 

 the mesonephric tubules have not attained their full develop- 

 ment. It is possible, however, to make out most of their 



post cardinal V 

 coelom 



mesonephric 

 duct 



mesonephric 

 tubule 



developing capsule 

 and glomerulus 



mesentery 



Fig. 53. — Drawing from transverse section of four-day chick to show meso- 

 nephric tubule and duct. For the location of the area drawn consult Fig. 46, F. 



fundamental parts (Fig. 53). The tubules lying in the ven- 

 tro-lateral portion of the mesonephros have been longest estab- 

 lished and are somewhat more advanced in development than 

 those lying in the dorso-mesial portion. Nearly all of the 

 tubules have become elongated and somewhat coiled. At one 

 end they open into the mesonephric duct or a diverticulum of 

 the duct which acts as a collecting tubule. At their other end 

 a cluster of closely packed cells indicates the place at which 

 the capsule and glomerulus will appear. The glomeruH develop 

 very rapidly. Circulation is usually established in them by 

 the fifth day. From this time until about the eleventh day 



