120 WALTER HEAPE. 



Such is the condition of the notochord during Stage h. At 

 the close of Stage j, however, the whole of the notochord, 

 except at the immediate anterior end, backwards to the ninth 

 protovertebra, is isolated as a rod of varying size and shape 

 (figs. 46 and 47). Behind the ninth protovertebra it becomes 

 band shaped and continues in this form, still distinct from the 

 hypoblast, for some distance behind the last (fourteenth) proto- 

 vertebra. It then again assumes the form of a rod, although 

 of much larger size than in the anterior region, in the centre 

 of which a lumen may here and there be seen, and joins the 

 anterior end of the primitive streak becoming thus connected 

 there with the epiblast, hypoblast, and lateral mesoblast 

 (fig- 50). 



The phenomena I have here described, viz. : (1) the presence 

 of a mass of primitive undifferentiated hypoblast in the 

 median line (Stage n) ; (2) its reduction to a thin, even single 

 layer of cells (Stages d, e, and v), and (3) the conversion of 

 those cells into the notochord (Stages g, h, and j) ; these 

 phenomena, in my opinion, indicate without doubt that this 

 organ is of hypoblastic and not of mesoblastic origin. 



Further, during the isolation of the notochord, (a) the 

 appearance, vague though it be, of an arc of notochordal cells ; 

 (b) the fact that the isolation of the solid rod or band com- 

 mences at the two sides and gradually extends across the 

 median line (figs. 39 — 42) ; and (c) the occasional appearance 

 of a lumen in this rod, — these appearances indicate that it is 

 formed in the same manner as the notochord of Amphioxus, 

 that is to say by the ingrowth of the lateral hypoblast and the 

 constriction of the axial mass of primitive hypoblast cells. 



I have already discussed the views of other observers upon 

 this subject (No. 8) and need not again refer to them. 



Figs. 39 and 40 are especially interesting in regard to the 

 isolation of the notochord. In both these drawings the 

 process of isolation is shown taking place ; in both the noto- 

 chordal tissue is in the form of a pair of knobs connected by a 

 median more slender portion; and in both cases when the 

 notochord is actually isolated it will be isolated as a band of 



