52 W. B. SCOTT AND HENRY F. OSBORN. 



formation of the organ, and is not^ as in Rana temporariay 

 wholly expended in forming the upper layer of the alimen- 

 tary canal. To return to Triton, it is well to notice here 

 that the upper boundary of the alimentary canal is formed 

 by the cells which will give rise to the notochord, and that 

 the latter at this period actually contains part of the lumen 

 of the canal. 



Following the notochord into the succeeding stage, we 

 find no marked changes (PI. IV, fig. 6). The section taken 

 from the middle region of the embryo presents much the 

 same appearance. From this we infer that in common with 

 the other organs, the notochord develops more rapidly for- 

 wards, and that the backward development is a slow one, 

 for in Stage c the notochord is but little more advanced in 

 the middle region of the embryo than it is in the an^^^erior 

 region in the preceding stage. The primitive features 

 pointed out above remain constant. 



Unfortunately there is a gap in our sections here, at least 

 we have none by which we can trace the histological changes 

 from the simple fold of hypoblast cells in Stage c, to the 

 firm rod of radiating cells in the latter part of Stage D. 

 There is no evidence of their splitting into two cells deep 

 previous to this result as in the Lamprey and the Elasmo- 

 branchs. The exact process beyond the ascertaining of this 

 point is of little real importance. 



In Stage d (PI. IV, fig. 7) the relations of this organ are 

 not much altered, it still impinges against the epiblast 

 above, and partly bounds the alimentary canal below, but 

 the continuity with the hypoblast has been broken oflT, and 

 the line of demarcation is plainly marked by the different 

 character of the cells. The notochordal cells are subquadrate 

 in shape, about twelve in number in a transverse section, 

 and are arranged around a centre of their own. In other 

 words, the notochord is now an independent body ; at its 

 sides below are the long narrow hypoblast cells which par- 

 tially enclose it, and above are the mesoblast plates fully 

 formed, which, however, show no tendency to sur- 

 round it. 



The notochord is now larger than at any subsequent stage. 

 In its formed or permanent condition, it persists as a close 

 granular mass in which we can sometimes detect cell 

 division, sometimes not. (See PI. V, fig. 8; figs. \% 

 and 13.) In Stage e an ingrowth of hypoblast below, cuts 

 off" its connection with the alimentary canal. In a much 

 later period. Stage m, it has a vacuolated appearance ; a 

 branching network of connective tissue sup})orting promi- 



