^ro JORDAN. [Vol. VIII. 



soon diminishes, whether by division or by actual loss of sub- 

 stance I do not know; but the effect is to produce cells of 

 about the same dimensions as the mesoblast cells that lie on 

 either side and often come into contact with the notochordal 

 cells (Figs. 56, 57). In many cases, and particularly in the 

 more anterior and posterior portions of the embryo, the noto- 

 chord begins to be formed while the connection of entoblast 

 and mesoblast still persists in the immediate neighborhood. 

 It thus comes about that both notochord and mesoblast take 

 their origin simultaneously from the same mass of entoblastic 

 cells, and the boundaries of the different elements are conse- 

 quently hard to define (Figs. 59, 64). The notochord, like the 

 mesoblast, becomes separated from the entoblast in the middle 

 of the embryo while the connection is still intact in the more 

 posterior portion (Figs. 60, 61). The small size of the meso- 

 blastic cells, as compared with the notochordal cells in this 

 region (Figs. 60, 61), renders impossible any confusion as to 

 the origin of the notochord at this point. 



The notochord is formed along its posterior portion to an 

 unknown and probably varying extent by "invaginate ento- 

 blast," and in its more anterior portion by "yolk-entoblast." 

 Posteriorly the chord runs into the fused mass of cells known 

 as the primitive streak, and anteriorly into a body of cells poorly 

 defined, but probably entoblastic (Fig. 64). 



Since Schultze's ('88) observations were made upon Rana 

 fiisca and not upon a Urodelan amphibian, I do not feel called 

 upon to criticise his results. I can merely remark that the 

 large number of cells in the dorsal mid-line of Rana is quite 

 different from the condition existing in the newt. It is 

 possible that this large number of cells may tend to obscure 

 somewhat the real course of events. I must also point out 

 the fact that certain of Schultze's figures (as Fig. \2a) do 

 not forbid the assumption that in Rana, as in Dieinyctylus, the 

 entoblast plays an important part in the formation of the 

 notochord. The fact that both notochord and mesoblast form 

 simultaneously from the same region of entoblastic cells has 

 also doubtless its share in producing delusive appearances. 



August, 1892. 



