Geo. C. Price 129 



tubules are J'onneil as evaginations of tlie dorsal walls of the nepliro- 

 tomes. However, it is not possible to tell where the tuljules thus formed 

 end, and the others begin.. Figs. 22 and 2-i represent transverse sections 

 through tubules of the two regions in a younger eiuljryo and Figs. 25 

 and 26 represent corresponding tubules in an older eml)ryo. The resem- 

 blance in the appearance of the tubules from the two regions is apparent, 

 notwithstanding the fact that there are important difit'erences. 



In Fig. 26 the thickening marked x forms ultimately the membrane 

 immediately surrounding the glomerulus; the glomerulus itself appearing 

 in the angle between this and the tubule. In Fig. 25 there is a similar 

 thickening, a% but here it later entirely disappears, the tulnile retains 

 the characteristics of a pronephric tubule throughout life, and no glomer- 

 ulus is ever formed. The attention was first called to the similarity of 

 the two kinds of tubules in the above particular while searching for 

 indications of the formation of glomeruli in the anterior part of the excre- 

 tory organs. It is here given for what it is worth, although it might 

 be interpreted as suggesting that formerly the anterior tubules as well 

 as the posterior were provided with glomeruli. 



By the disappearance of the nephroccels in the anterior part of the body, 

 and by their 2>ersistence and subsequent separation from the splanch- 

 nocoel in the posterior part, the excretory system becomes quite early 

 divided into two regions; but for a long time it is impossible to deter- 

 mine the exact lioundary between the two, owing to the fact that there 

 is an intermediate region of a few segments, in wliieli the nephroccels 

 have neither merged with the splanchnocoel nor have they Ijeen cut off 

 from it, and there is nothing to indicate which their ultimate fate will be. 

 Moreover, in front of this the nephrocot'ls do not end abiiiptly but be- 

 come gradually shallower and shallower. Later the boundary becomes 

 definite, Init it is not constant for different individuals, nor is it always 

 constant for the two sides in the same individual. The l)est that can be 

 said is that it is in the neighborhood of the thirtieth segment. 



Corresponding to the above described regions there are two Idnds of 

 excretory tubules, those in the anterior region retaining the character- 

 istics of pronephric tubules throughout life, while those in the posterior 

 region assume the characteristics of mesonephric tubules. These terms 

 would be employed in speaking of them if it were not that it would lead 

 later to the contradictory expression, "" the mesonephric tubules of the 

 pronephros." For this reason the first will be spoken of vsimply as the 

 open tubules, and the second as the closed tubules. 



Of the closed tubules those in about the last eighteen or twenty seg- 

 ments degenerate (the number not being constant), wdiile the remainder 



