236 RITTER 



separated out distinctly from the main bundle, and the test matter with 

 its scattered test cells can be clearly seen, fully enveloping the fibers. 

 It can be confidently asserted that, in most cases, no lumen is present 

 in the ultimate rootlets in which blood may flow.' 



In all probability these are all developed at first as ectodermal ves- 

 sels, and the ectodermal layer and lumen of the vessel become sec- 

 ondarily obliterated. It is difficult to see by what other method the 

 conditions could be reached. The only direct confirmatory evidence 

 that I have been able to get, however, is the fact that in one rather 

 small specimen from which most of the peduncle was wanting, because 

 of having been broken off, as I at first supposed, the rather short re- 

 maining piece was found to contain an outpocketing of the ectoderm 

 of the mantle, with the epithelial layer showing very distinctly. No 

 muscle fibers were present, however, in this instance, and this fact to- 

 gether w4th the failure to find, on closer examination, evidence that 

 the peduncle had been broken, leads me to believe that this was an in- 

 cipient peduncle into which the muscle fibers had not yet penetrated, 

 and from which the ectodermal layer had not begun to degenerate. 

 Before leaving this subject, so well deserving further study on young 

 animals, I would draw particular attention to the appai-c7tt gradual 

 transition of the test cells themselves, into the muscle fibers, as shown 

 in p1. XXVIII, fig. 17. This figure is drawn from a peduncle near the 

 body and might be duplicated from any specimen. It is, of course, 

 impossible to afiirm that there has been a production of muscles fibers 

 from test cells, as the adult structure here suggests. In view, however, 

 of the fact that the test cells are now known to be of mesenchymatous 

 origin it is not difficult to suppose that the development has taken 

 place as the structure actually found indicates. Muscle fibers might 

 very well be formed, probably are formed, from mesenchyme cells in 

 the vessels while these still exist. But the mesenchyme cells migrate 

 through the ectoderm to become test cells. With the disappearance 

 of the ectoderm, then, mesenchyme and test would be in direct con- 

 tact, and what would be essentially a transition from test cells to mus- 

 cle cells would then be possible. 



^On consulting the classical work of Lacaze-Duthier,i874, on the Molgulidae,! 

 find the following concerning the villosities of the test. One may make out the 

 nuclei of the walls of the canals, the author says, " mais on ne distingue jamais 

 aussi bien qu'apres avoir employe les imbibitions ou les autres manipulations 

 histologiques, ces cellules regulierement poljedriques." It is possible therefore 

 that further study of the root hairs of Rhizomolgula on fresh and specially pre- 

 pared material will bring out the epithelium, but from my experience it seems 

 hardly probable. 



