Geo. C. Price 



119 



of ages corresponding to those in whicli the above structures are figured 

 1 have been unable to find any such system of tubules, although on 

 ■account of their size it would seem impossible to overlook them if present. 

 In Dean's figure 13-i the parts marked pronephric tubules bear a strik- 

 ing resemblance, even down to the fine details, to the ventral part of 

 the muscle segments as seen in sagittal sections of an embryo of about 

 the same age as the one from which the above figure was taken, and in 

 the sections there is a comparatively large amount of connective tissue 

 between the muscles which corresponds well with the space between the 

 so-called tubules. Moreover, the muscle segments are the only seg- 

 mental structures that at all correspond in size to the ones in question. 



The youngest embryo studied has one hundred and one myotomes on 

 the one side and one hundred and two on the other, a number fully 

 equalling that of the muscle segments in the adult. There are seven 

 or eight pairs of forming gill slits, only five of which could be made 

 out from the surface view. 



Fig. 1. — Section through the fifty-second segment of the youngest emhryo studied. 

 a, aorta ; ch, notochord ; g, spinal ganglion ; Im, lateral mesoblast ; my, myotome ; 

 nc, nephrocoel ; nt, nephrotome ; scl, sclerotome. There is an artificial break, shown 

 best on the right of the figure, separating the myotome from the nephrotome and 

 extending into the sclerotome. The sclerotome is seen to be in connection with the 

 myotome, and on the right the lateral mesoblast is continuous with the nephrotome. 



In this embryo there are neither excretory tubules nor excretory ducts 

 in the true sense of the word, but there is an extensive system of nephro- 

 tomes from which both tubules and ducts are derived. These extend 

 certainly from the thirteenth segment to the seventy-fourth on one side 

 and to the seventy-fifth on the other, and it is possible they may begin 

 even farther forward than the thirteenth. From the anterior end back 

 to the neighborhood of the fiftieth segment they are all practically in 

 the same stage of development, but from here on they become gradually 

 less well developed until, in the last few segments, they appear to be 

 just forming. 



An idea of the relations of the nephrotomes to other parts may be 

 had from Fig. 1, which represents a section passing through the fifty- 



