510 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [56] 



vestigated these structures in other forms microscopically, and shown 

 them to contain sensory end-organs of a highly specialized character. 



Besides the foregoing nervous dermal structures, there are present upon 

 the skin of the larvae of some forms a very singular type of structures 

 which have apparently not been much studied. These are the goblet 

 cells of the skin. Their function is probably not sensory, but secretory, 

 pouring out a mucous substance over the skin. In the early stages of 

 young fishes generally which I have studied they are usually absent; 

 in fact, I have never met with them except in the larvae of those Sal- 

 monoids with large ova and embryos. They are apparently unicellular, 

 (Gegenbaur); at least this is their appearance in the young. of Salmo 

 and OncorJiynchus. In form they are globular, with a wide, trumpet- 

 shaped mouth or external extremity, which apparently represents the 

 efferent opening of these unicellular glands. They are very numerous, 

 imbedded in the epithelium of salmon embryos, and are found all over 

 the head and body, extending even over the whole of the yelk-sack^ 

 which is thickly studded with them. 



14. —Development of the notochord. 



The disputed question of the particular layer from which this organ in 

 the Teleosteau embryo is derived I am unable to settle definitely. Some 

 authorities hold that it is derived from the ventral edge of the neural 

 keel by delamination, being split off from before backwards as a chord 

 of cells. It is not certain, however, that it may not originate fr»m the 

 lower layer or hypoblast, and not from the neural keel ; at any rate, a 

 more exhaustive study of the subject is still required before a definite 

 conclusion can be reached in regard to the origin of this organ. It is 

 developed before the intestine acquires a lumen, and while that structure 

 is still a solid median band of hypoblast cells, lying just below the i>rim- 

 itive chorda. Transverse sections show it as a distinct, slightly depressed 

 rod or cylinder, which extends from just behind the infundibulum, be- 

 low the medulla oblongata, to the caudal plate or mass of cells, in which 

 its posterior extremity is completely lost. The cells of which it is at 

 first composed are not distinguishable from those of the mesoblastic 

 muscular and splanchnic layers on either side of it, but by the time of 

 the closure of the blastoderm over the yelk its presence may be very 

 easily detected in the living embryos as well as in transverse and longi- 

 tudinal sections of the same, having been distinctly segmented off from 

 the adjacent structures in the vicinitj. With the downgrowth of the 

 neurula to form the carina, the mesoblastic tract or layer has undergone 

 changes of development, or rather localization, on either side of the mid- 

 dle line, by which the muscular layer becomes completely separated into 

 two lateral longitudinal masses, with the chorda lying between them, and 

 with only a very thin stratum of splanchnic mesoblast underlying, from 

 which it would appear that the aortic and venous trunks of the body 

 and the peritoneum arise at a later period. This splanchnic layer is iu 



