140 



The account given by Dr. Klaatsch of the histogenetic develop- 

 ment of the horny fibrils of Elasmobranchs is incomplete, since he 

 seems not to have studied the earlier stages. He seems to have 

 relied on sections alone, by which means it is impossible, owing to 

 the extreme fineness of the structures, to perceive the first develop- 

 mental stages. It is scarcely to be doubted that the histogenetic 

 development is the same in Salmo as in Heptanchus. In my own 

 description of the development in the former, it was shown that the 

 horny fibrils are the product of the fine cell-processes of the mesen- 

 chyme, which at first form a loose meshwork in the fin-folds. It is 

 only in later stages, when the mesenchyme has proliferated to a great 

 extent, that the cells wrap themselves so intimately around the fibrils, 

 giving the appearance, described by Dr. Klaatsch, that the fibril is 

 an intracellular secretion. 



As there are no figures given in Dr. Klaatsch's paper to illu- 

 strate the development of the fin-rays, I find it difficult to under- 

 stand his meaning. I am able to explain his account of their develop- 

 ment only by the conjecture that the material at his disposal was 

 not in sufficiently good histological condition to show all details clearly. 



Nothing can be more evident than the fact that the cells which 

 give rise to the fin-rays are of mesenchymatic origin ; in the beginning, 

 however, the substance of the rays is formed in close contact with 

 the basement membrane by the outer surface of the mesodermal cells. 

 There are no perceptible breaks in the continuity of the membrane. 

 In some cases one might be led with Dr. Klaatsch to consider the 

 fin-rays as "locale leistenartige Verdickungen der Basalmembran". 

 That the two structures are entirely independent, however, is proved 

 by the fact that they are frequently torn slightly apart in the pre- 

 paration of sections. 



Later on, the ray sinks somewhat deeper into the underlying tissue, 

 losing contact with the basement membrane and becoming entirely 

 surround by osteoblastic cells. In a growing ray, then, the distal 

 portion is always in contact with the ectoderm, while the proximal 

 portion is surrounded on all sides by formative cells. 



In figure 2 there is represented a section through the distal portion 

 of a ray with its osteoblasts and the overlying ectoderm. The base- 

 ment membrane is clearly defined and perfectly continuous. There is 

 however a break in the ectoderm, so that the lower layer is split off 

 from the rest at a point opposite the ray, and remains in contact 

 with the basement membrane. It is this appearance, I think, which 

 has led Dr. Klaatsch to say: ''sie (die Strahlen) hängen gleichsam 



