THE LATER DEVELOPMENT OF THE FROG 197 



The first indication of this is a horizontal fold of the splanchnic 

 or medial wall of the dorsal ccelom just opposite the second 

 nephrostome. This fold appears at about 4.5 mm. and its 

 development is in general parallel with that of the pronephros. 

 It soon extends the entire length of the pronephric region and 

 becomes considerably elevated, projecting freely into the 

 coelom opposite the nephrostomes (Fig. 72). In it vascular 

 spaces soon appear, some of which form the long convoluted 

 vessels of the glomus proper, while others go to form a vessel, 

 connecting with a branch of the dorsal aorta, which is ap- 

 parently one of the segment al arteries passing ventrally from 

 the aorta. Later this section of the body cavity is cut off, as 

 the pronephric chamber, by the lateral projection of the lung ) 

 (Fig. 72, B) which carries a fold of peritoneum across to the / 

 peritoneum covering the pronephros, with which it fuses for a 

 short distance. This pronephric chamber remains open into the 

 body cavity both anteriorly and posteriorly to the lung region. 

 A definite pronephric capsule is formed early from two 

 sources. The ventro-lateral walls of the myotomes, which we 

 have seen give rise elsewhere to mesenchyme, here, in the pro- 

 nephric region, evaginate over the dorsal and lateral surfaces of 

 the pronephros, and meet folds coming up from the somatic 

 layer of the lateral plate (about 6 mm.). These folds form a 

 definite connective tissue layer enclosing the pronephros and 

 the pronephric sinus of the posterior cardinal vein. 



The pronephros reaches its full development in the larva of 

 about 12 mm., when it consists of a large mass composed of the 

 coiled proximal end of the pronephric duct and the three 

 tubules, each of which has acquired blind tubular outgrowths, 

 the whole mass interpenetrated by the vascular sinus of the 

 posterior cardinal vein. In the larva of about 20 mm. the 

 pronephros commences to degenerate. At this time the pro- 

 .lephric duct becomes closed just back of the pronephros; the 

 tubules become variously dilated and constricted, breaking 

 into irregular sections and gradually disappearing (Fig. 73, C). 

 The nephrostomes approach one another and finally meet, 

 opening into a common cavity known as the common nephro- 



