THE CHORDA DORSALIS. 97 



tiply so rapidly, that in a short time the substance of the chorda is com- 

 posed almost entirely of them, the granular matter being reduced to a 

 small quantity of intercellular substance. As the cells increase in size, 

 clear, flat, vesicular nuclei are shortly developed within them ; they are 

 sometimes central, sometimes parietal, never granular, or provided with 

 a nucleolus. Around the cellular substance of the chorda, a closely 

 fitting sheath is early developed j it appears first in the form of a clear 

 pellucid fluid, but subsequently assumes a fibrous character, probably from 

 the transformation of cells which are shortly developed in it. In the chorda 

 of the Triton Lobatus, a granular substance similar to that in the chorda of 

 the toad, is first observed; but instead of being scattered irregularly, the 

 granules have a somewhat circular arrangement, being disposed like so 

 many rings around the chorda. As in the toad, so also in the triton, these 

 granules shortly disappear, and are replaced by cells, the formation of which 

 presents itself first at the cephalic extremity, and rapidly extends through 

 the whole length of the chorda. Like the granules which preceded them, 

 these cells in the triton observe an orderly arrangement ; each cell having 

 a size equal to the diameter of the chorda, and, being flattened by the 

 pressure of the adjoining cells, contributes in giving to the chorda 

 the appearance of being composed of a linear series of rings enclosed 

 within its fibrous sheath. The cells lie close to each other, very little 

 intercellular substance being present. At a later period of the larval 

 life of the triton, this regular arrangement of the cells is somewhat modi- 

 fied. The diameter of many parts of the chorda, instead of being occupied 

 by only one nucleated cell, now presents two or three of about equal size. 



Concerning the origin of the finely granular material of which the 

 chorda is at first composed, and from which the cells are developed, Vogt 

 is of opinion that it is furnished by embryonic cells. He considers that 

 along the line to be subsequently occupied by the chorda dorsalis, are de- 

 posited embryonic cells similar to those from which all other organs of the 

 embryo are formed : that the membranous walls of these cells shortly dis- 

 appear, and that the cell contents thereby liberated constitute the granular 

 matrix or cytoblastema from which the second set of cells of the chorda 

 are formed.* Kolliker,f however, appears to be of opinion that the cells 

 of the chorda dorsalis are produced directly from the primary embryonic 

 cells which simply enlarge, and at the same time lose the granular matter 

 which they originally contained. According to Prevost and Lebert,{ again, 

 they are formed from the enlarging nuclei of the embryonic cells. The 

 development of the chorda dorsalis in the embryo of the salmon (Corego- 



* This is in accordance with his opinion, again to be mentioned, that none of the 

 embryonic tissues are developed directly from the first embryonic cells, but that these cells 

 invariably resolve themselves into a secondary blastema, out of which are formed new cells, 

 which give rise to the tissues of the embryo. 



t An. des Sc. Nat. 1846, p. 92. J An. des Sc. Nat. 1844, p. 206. 



H 



