. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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-aceti
. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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,
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