258 BULLETIN OF THE 



disposition of the tubes (Fig. 62). The convolution in this stage is 

 no longer confined to the common trunk, the nephrostomal tubules un- 

 dergoing slight contortion (Figs. 63-G5). 



I have determined the positions of the pronephric structures to the 

 somites in these later stages by their relations to the spinal ganglia. 

 The first and second nephrostomes lie very nearly in the same transverse 

 plane as the first and second spinal ganglia respectively. In the young- 

 est larvaj of this stage the boundaries between the myotomes may still 

 be made out in transverse sections, and the nephrostomes are then found 

 to lie beneath myotomes III. and IV. It is probable that in later 

 stages as well two myotomes occur in front of the first spinal ganglion. 



The duct after leaving the pronephros pursues a nearly straight course 

 backwards to the cloaca. In the larvae of this stage, the post-anal gut 

 has atrophied, and the ducts open into the intestinal tract just at the 

 point where it bends downward toward the anus or cloacal aperture. 

 The outlets of the two sides of the body are quite widely separated, 

 never opening into an unpaired median depression in the dorsal roof of 

 the cloaca, as is the case in the corresponding stage of Rana. The out- 

 lets of the segmental ducts are situated between the eighteenth and the 

 nineteenth spinal ganglion, which would correspond to somite XX. or 

 XXI. Their position is, then, the same as in the preceding stage. 

 (Compare page 254.) 



In the series of embryos included under Stage V., it was shown that 

 the walls of the pronephric tubules became gradually thinner as the ani- 

 mal developed. In the pronephridia of the present stage the same pro- 

 cess has been continued, and the cells are frequently so reduced in 

 thickness that the nucleus appears to be in contact with the basal as 

 well as the superficial, or inner, surface of the cell. Occasionally tubes 

 occur whose walls are so thin that each nucleus causes a protuberance 

 into the lumen of the tube. But wherever the thickness of the epithe- 

 lium exceeds the diameter of the nucleus, it is to be noticed that the 

 latter lies close to the inner surface of the tube, whereas the yolk 

 spherules are accumulated in the basal portions of the cells. The yolk 

 spherules are much less numerous than in the preceding stage. In 

 many cells they are wholly wanting, and in all they now form a much 

 less prominent constituent than the cell protoplasm. 



The nephrostomes present no new features of interest in this stage. 

 Most of the pronephric tubules contain more or less pigment, which is 

 Tisually accumulated in irregularly distributed dark patches. In one or 

 two instances I have had a fair degree of success in dissecting out the 



