470 EDWIN S. GOODRICH. 



unable to say ; but there can be uo doubt tliat it is formed 

 very early indeed. It is clearly present, for instance, in 

 Scyllium embryos 14 mm. in length, long before either 

 denticles or dermal rays have begun to develop. Once laid 

 down the basement membrane remains, as far as can be seen, 

 continuous and unbroken throughout embryonic life. Of any 

 migration of scleroblasts from the epidermis there is not the 

 slightest trace, and I have no hesitation in stating that it 

 does not occur in the embryos I have examined.^ 



After the deposition of the basement membrane, which 

 stains dark blue with nigrosin, the actiuotrichia begin to 

 develop in those regions of the, at first, continuous median 

 fin which are destined to give rise to the adult fins. Almost 

 simultaneously ceratotrichia begin to develop in the paired 

 fins. Since the fins grow at their free edge, the same section 

 shows various stages in the development of the ra3^s according 

 as the parts nearer or farther from the body are examined. 

 Near the edge itself the first origin of the rays can be traced. 



Here the secreting cells are seen to lie close under the epi- 

 dermis, and to deposit the horny substance, if not in actual 

 continuity with, at all events in immediate contact with the 

 basement membrane, on to the under surface of which they 

 cling (figs. 8, 10, and 12). Thus the first rudiments of the 

 dermal rays appear to be formed on the outer surface of the 

 scleroblasts, which range themselves in parallel rows. Soon 

 they seem to insinuate themselves between the ceratotrichia 

 and the basement membrane, so that the ray becomes gradu- 

 ally separated off from the membrane as it grows older, and 

 sinks farther and farther into the mesoblastic tissue (figs. 9, 

 11, and 7). In the meantime the young ray becomes sur- 

 rounded by a sheath of secreting cells, and increases in bulk 

 by the concentric addition of layer upon layer of fibrous horny 

 substance. Occasionally the younger portion of the ray may be 

 seen to be connected with the basement membrane by fine 

 strands of cuticular substance, which, like the membrane and 

 the ray itself, stains dark blue with nigrosin (fig. 9). 



' HarribOii (15) oblaiiis the same results in Teleosts. 



