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



The Notochord. 



In our description of the hypoblast, we considered the 

 layer as classed under two heads, the invagination hypoblast, 

 and the yolk hypoblast ; it is with the former that the 

 development of the notochord is concerned. The cells lying 

 during Stage b between the mesoblast plates may be con- 

 sidered the primitive notochordal cells. 



The first indication of the growth of the notochord in 

 Triton (see Plate XX, fig. 5), is the tendency of the cells 

 to take a radiated arrangement. We may now at the out- 

 set, point out three prominent features. • First, the hypo- 

 blast consists of a single layer of columnar cells running 

 from the epiblast above to the rlimentary canal below. 

 Second, these cells may be identified with the broad band of 

 invagination cells which in Stage a were all in contact with 

 the epiblast. They have been bent down by the ingrov/th 

 of the mesoblast above. Third, these cells are directly con- 

 tinuous at the sides with the yolk hypoblast. 



In the Lamprey,^ Petromywn planeri, the relations of 

 the hypoblast at this point to the epiblast and mesoblast are 

 practically the same. There is the same close and broad 

 contact with the epiblast, and the cells are of the same 

 relative size. Here, as in Triton, the primary or invagina- 

 tion cells are alone concerned in the origin of the notochord. 



In the Frog {Rcma temporaricbf the primitive condition of 

 the notochord is a great cubical mass of small cells, con- 

 fluent with the epiblast above, and with the mesoblast at 

 the sides. These do not all enter into the formation of the 

 notochord, however, for at the time this organ begins to be 

 constricted off, the lower cells form a hypoblastic lining to 

 the alimentary canal. Gotte's account of the first appear- 

 ance of the notochord in the Frog [Bomhinator igneus) 

 differs widely, owing to the fact that he has mistaken the 

 upper hypoblast cells for the mesoblast. 



In the Elasmobranch Fishes^ the arrangement is analo- 

 gous, for the whole layer with the exception of a thin line of 

 cells over the alimentary canal, enters into the notochord. 

 The cells are at no time so widely in contact with the 

 epiblast as in Triton ; so the change preceding the formation 

 of the notochord consists, first, in the lengthening, and then 

 splitting of the cells into two lines placed end to end. The 

 lower line thus formed is, however, mostly absorbed in the 



' Vide E. Calberla, loc. cit. 



2 Yi^g E. Calberla, loc. cit., p. 260. 



2 Vide Balfour, loc. cit., p. 93. 



