141 



Fip. 2. Section through fiu-ray of the caudal fin of S. sahir, about 2 cm long. 

 ect. ectoderm; bm. basement membi-ane; J. r. fin-ray; h. f. horny fibril; sp. artificial 

 split in ectoderm. 



an epithelialen Stielen in die Bindegewebsmasse der Flosse hinein. 

 Dabei werden sie allseitig von Skleroblasten umhüllt". That the break 

 in the ectoderm is, however, purely artificial, is proved by the fact 

 that it is by no means always present, and is less likely to be found 

 after fixation in sublimate than in the chrom-osmo-acetic mixture. 

 Longitudinal sections through a ray demonstrate clearly the cause of 

 the break; it is merely due to the resistance offered to the section- 

 knife by the hard bony substance. At the outer end of the ray, where 

 it is still thin, and offers little resistance, the break ceases; proxi- 

 mally where the ray is separated from the ectoderm and entirely sur- 

 rounded by mesenchyme the break also ceases. The basement mem- 

 brane in such cases, nowhere loses its continuity and an in-wandering 

 of the lower layer of ectoderm, which is split off from the rest does 

 not take place. The osteoblasts which, proximally, cover the exterior 

 of the rays, are seen in longitudinal sections to be quite independent 

 and distinct from the ectoderm, while cross sections show them to be 

 continuous with the mesoderm. 



As regards the contribution of ectoderm to the rudiments of other 

 bony structures, such as operculum and shoulder girdle, it seems to 

 me certain, that here, as well as in the fins, the apparent breaks in 

 the continuity of the basement membrane in the vicinity of the bones, 

 are due merely to the fact that in such places the plane of section is 

 not perpendicular to the surface. While, of course, this is difficult to 

 prove in all cases, in a great many it is certain. For instance, cases 

 may be observed, in which sections cut in one plane show the con- 

 tinuity of the cells of the two layers, while sections cut in a plane 

 perpendicular to the first show a fold in the ectoderm in precisely 



