139 



after fixation in Flemming's chrom-osmo-acetic mixture. Carmine does 

 not bring it out so clearly. In the paired fins the same sharply de- 

 fined line is present. Fig. 4, PI. VII of Dr. Klaatsch's work represents 

 older stages equally as well as the younger for which he has especially 

 intended it. Here the epidermis is sharply marked off from the under- 

 lying tissue and the cells of the deepest layer are decidedly columnar. 

 It is only in sections which cut tangentially the anterior or posterior 

 edges of these fins, that I can find, at the free border, the apparent 

 breach in the basement membrane, and continuity of the ectoderm 

 with the mesenchyme at that point, described and figured by Dr. 

 Klaatsch, Fig. 5, PI. VII. Such places are more pronounced in the 

 pectoral fin, which makes its appearance at a much earlier stage than 

 the pelvic, at a time when the basement membrane is less well defined 

 and smooth. The explanation of the figure is however not difficult, 

 for at these points the membrane must be cut obliquely, and hence 

 there results a slight overlapping 

 of the two layers. This is always 

 much more marked in thicker sec- 

 tions, but careful focussing with high 

 powers, best with oil immersion, 

 will often show that a line of de- 

 marcation is present, where with 

 weaker magnification , continuity 

 might be supposed to exist. A very 

 striking proof that the two layers 

 are entirely distinct from one an- 

 other, and that no crowding of cells 

 out of the ectoderm into the under- 

 lying tissue can take place, is 

 afforded by many of my prepara- 

 tions in which the ectoderm of the 

 fin has been lifted up from the 

 mesenchyme through osmotic action. 



Fig. 1. Section through dorsal fin of Si irideus, 1,5 cm long. ect. ectoderm , mes. 

 mesenchyme of fin, h. f. horny fibril. 



Figure 1 represents a cross section through the median fin of such 

 a specimen. Here the fibrils lif are already formed but the fin- 

 rays are not yet present. The same relations between ectoderm and 

 endoderm may be followed through the whole series of sections, and 

 moreover may be found in embryos at various stages. 



