THE URO-GENITAL SYSTEM 371 



by itself directly to the exterior. It may then be supposed 

 that for the better disposal of the excretory fluid the separate 

 openings became connected by a groove which continued to 

 the side of the cloacal orifice and deepened posteriorly into a 

 trough, from which by a further continuation of the process 

 an internal tube would be formed, 

 opening either at the margin of the 

 cloaca or just within it. The meso- 

 dermic anterior portion may be the 

 result of the fusion of the outer 

 ends of the succesive nephridia, 

 each one contributing that portion 

 belonging to its own somite. 



Typical pronephridia (Fig. 107, 

 A), the units of the pronephros, 

 closely resemble the one given in 

 the theoretical description above. 

 They .possess at the inner end 

 ciliated nephrostomes and show a 

 greater or less tendency to coil, 

 suggesting a former condition of 

 considerable physiological effi- 

 ciency. Aside from this, they show 

 the beginning of a relationship es- 

 sentially vertebrate, and carried 

 out in greater detail in the meso- 

 and meta-nephrotic systems, fu f ctional pronephros and de- 



, . . velopmg mesonephros. [After 



namely, an association with capil- MARSHALL] 



lary blood-VeSSels, enabling the v, ventricle of heart; t, truncus; 



nephridia to extract waste mate- l' OT Sl 

 rial directly from the blood. This 

 association is here very slight, Wolffian duct - 

 and consists of segmentally arranged tufts of capillaries, glo- 

 meruli, which protrude into the ccelomic cavity and form 

 rounded elevations covered by the peritoneum. These are 

 located opposite the nephrostomes, and the excretory fluid, 

 which passes from the glomefuli to the ccelomic cavity, is 



FIG. 106. Frog tadpole with 



w, 



