STRUCTURE OF FOUR-DAY CHICKS 147 



cords thus formed continue to extend caudad beyond the 

 pronephric tubules and soon become hollowed out to form open 

 ducts. When they eventually reach the level of the cloaca they 

 turn ventrad and open into it. 



The significance of the rudimentary structures in the chick 

 which represent pronephric tubules, can be most readily 

 understood by comparing them with fully developed and func- 

 tional pronephric tubules. Figure 52, B, shows the scheme of 



1 mesonephric 



duct 



nephrostome 

 glometulus 



FIG. 52. Drawings to show nephric tubules. A, drawing from transverse 

 section through twelfth somite of 16 somite chick to show pronephric tubule. 

 (After Lillie.) JB, schematic diagram of functional pronephric tubule. (After 

 Wiedersheim.) C, drawing from transverse section through seventeenth somite 

 of 3o-somite chick, to show primitive mesonephric tubule; D, schematic diagram 

 of functional mesonephric tubule of primitive type. (After Wiedersheim.) 

 For a later stage of the mesonephric tubules of the chick see Fig. 53, 



organization of a functional pronephric tubule. The ciliated 

 nephrostome draws in fluid from the ccelom. As the fluid passes 

 the capillaries of the glomus, waste materials from the blood are 

 transferred to it. The nephric duct serves to collect and 

 discharge the fluid passing through the tubules. Vestiges of 

 a nephrostome opening into the ccelom appear in the pronephric 

 tubules of the chick (Fig. 5 2, A) but the tubules never become 



