130 Excretory Organs in Bdellostnina Stouti 



persist, and all except a small nnniber at the anterior end become 

 mesonephric tubules of the adult. 



Both of these points were noted in the previous paper, but it was 

 there stated that in the oldest embryo of stage B the tubules had dis- 

 appeared in the last nineteen segments. A subsequent very careful 

 examination proved this to be a mistake. They have disappeared in the 

 last eleven segments, but in the next eight they are still present in a rudi- 

 mentary condition. Their entire disappearance does not occur in this 

 region until quite a little later. 



In the earlier paper the statement was made that the series was 

 sufficiently complete to enable one to say that the mesonephric tubules 

 of the adult were derived from tubules which in the embryo arose as 

 pronephric tubules. The opinion has been expressed by some authors 

 that the evidence given was not sufficient to justify the assertion, and it 

 is not to be denied that, on account of the wide gap between stages B and 

 C, the objection may have been reasonable. But an examination of 

 specimens both older and younger than those before studied, as well as 

 of quite a complete series between B and C, has proved the truth of the 

 statement. In no case in the region of the mesonephros of the adult 

 is there the slightest indication of the disappearance of one set of tubules 

 and the appearance of another, but on the contrary the first tubules to 

 appear may be traced into what can be none other than the mesonephric 

 tubules of the adult. 



Wheeler ^ objected further on the ground that in a young specimen of 

 Myxine Mass found mesonephric tubules forming in the posterior part 

 of the body independently of the duct. As a matter of fact Mass " de- 

 scribes and figures two well developed Malpighian corpuscles, one with and 

 the other without a tubule, neither of which is in connection with the 

 duct. He does not state, however, whether these occur in segments in 

 which later mesonephric tubules are invariably present, and until this 

 is known it seems at least possible that the structures in question may 

 be degenerating instead of developing tubules. However it may be in 

 Myxine in Bdellostoma in a few cases very small, degenerating tubules 

 have been found far back in the body, independent of the duct. But 

 liere a comparison with other tubules in the same embryo, as well as 

 with embryos both older and younger, shows that these have been 



^Wheeler, W. M.: The Development of the Urinogenital Organs of the 

 Lamprey. Zool. Jahr., Vol. 13, Anat., 1899. 



■•Mass, Otto: Ueber Entwicklungsstadien der Vorniere und Urniere bei 

 Myxine. Zool. Jahr., Vol. 10, 1897. 



