PROCESS OF DEVELOPMENT IN OVA OF FISH. 531 



its inferior layer, large cells which are disposed, like those of the 

 Fowl, in irregularly distributed heaps. By degrees the inferior 

 surface becomes plane, and the part lying over the cavity then 

 appears to be composed of two layers of uniformly small cells. 

 The upper layer consists of a single tier of cells, but the lower 

 is composed of two or three tiers of cells. Further research 

 showed that the analogue of Remak's sensorial lamina was 

 developed from these two layers. This is here, therefore, as 

 in Batrachia, composed of two distinct rudiments. 



Large coarsely granular elements lie on the floor of the germ 

 cavity. The origin of these elements can scarcely be a matter 

 of doubt. The yolk of the egg of the Forella contains no mor- 

 phological elements from which they can proceed. No other 

 view can be held, therefore, than that they are the remains of 

 the segmented germ which, on the elevation of the latter from 

 the yolk, remain in part lying upon this, and have in part fallen 

 down upon it from above. We thus see the parts around the 

 germ cavity present relations analogous to those we have 

 already met with in the germ of the Fowl. 



Fundamental differences, however, do exist, and in order to 

 establish them I must enter into some comparative embryo- 

 logical details. Coste* long ago called attention to the fact that 

 the embryo of the Fish does not originate in the axis of the germ, 

 as occurs in Fowls, but along a part of the thickened border. 

 The relations that are here present may be readily understood 

 by the following conception. Imagine a small sphere of wax 

 placed upon a large wooden ball, and the former flattened out 

 into a disk with a thickened border; the disk continues to 

 expand, its thickened border constantly increasing in size till 

 it reaches the sequator of the wooden ball. And now the wax 

 cap may be conceived to enlarge still further, whilst the thick- 

 ened border becomes constantly smaller, till when the opposite 

 pole is reached it is reduced to the condition of a scarcely per- 

 ceptible annulus. The ball will then be almost completely en- 

 veloped by the wax cap. Relations exactly resembling the above 

 exist between the germ and the yolk of the egg of the Forella. If 

 the ova at various stages of development be hardened in chromic 



* Loc. cit. 



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