Ch. XIV] ORCxANS FROM THE MESODERM 157 



tract (Fig. 41). Presumably the pronephros is functionally 

 active at this time. The arrangement of the tubes of the 

 pronephros, and their relation to the common tube or prone- 

 phric duct, is shown in Fig. 48, C. The three nephrostomes 

 open into three collecting tubules, and these tubules have 

 elongated independently of one another. The first collecting 

 tubule is short ; the second is thrown into several turns and 

 opens into the pronephric duct a short distance from the first. 

 The collecting tubule from the third nephrostome opens some 

 distance behind the point of opening of the second. The seg- 

 mental duct is thrown into a series of turns between the first 

 and second collecting tubules ; and as it leaves the pronephric 

 region it takes at first a tortuous course, and then runs as a 

 straight tube backward to the cloacal opening. 



The posterior cardinal veins have appeared at this time, and 

 in the region of the head-kidneys these veins widen into a sinus 

 lying amongst the windings of the collecting tubules of the 

 pronephric duct. The glomus of each side reaching from the 

 region of the first to that of the third nephrostome, and lying 

 exactly opposite the nephrostomes, is well developed (Fig. 46). 



So far the description of the development of the excretory 

 system has been that given by Field. The same author adds : 

 " According to the account which at present receives the most 

 general acceptance, the pronephros first appears as an outfold- 

 ing of the somatopleure in the form of a longitudinal groove. 

 The anterior end of this groove is destined to become the prone- 

 phros, the remaining portion is constricted off to form the seg- 

 mental duct. Since the process of constriction advances from 

 before backward, stages may be found in which a completed 

 tube is continuous posteriorly with a mere groove of the soma- 

 topleure. In the anterior region the groove remains in com- 

 munication with the body-cavity, and grows down toward the 

 ventral surface of the embryo in the form of a broad pocket. 

 The slit-like peritoneal opening of this pouch closes through- 

 out the greater part of * its length, leaving, however, two or 

 three regions of incomplete closure, the fundaments of the 

 nephrostomes." 



" The nephrostomal tubules are formed by the fusion of the 

 walls of the pouch between two nephrostomes. The regions of 



