742 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



it is demonstrated that where the hypoblastic cells are not converted into the enteric arch 

 they proliferate to form the notochord anteriorly ; while the notochordal cells, nc, origin- 

 ating in this way merge posteriorly in the enteric roof (PI. IV. fig. 5c), precisely as they 

 unite in Elasmobranchs (No. 15, p. 683). 



The notochord arises then as a ridge, or median proliferation, of the hypoblast in the 

 posterior portion of the mid-trunk ; extending from that region, anteriorly, chiefly by a 

 progressive proliferation of hypoblast below, but doubtless to some extent, as already 

 mentioned, by a forward pressure of the hind part of the rod which is first formed. 

 Kolliker's view, that the notochord is continuous with the primitive streak (No. 81), 

 from which latter mass of cells the mesoblast arises and progresses forward, is consonant 

 with such a forward growth of the notochord in Teleosteans as we have indicated. 

 While the mesoblastic origin of the notochord is not generally accepted, there remains a 

 possible mode of origin which sections do not directly discountenance. If it is neither 

 formed from mesoblast nor hypoblast, it may yet be an axial differentiation of lower- 

 layer cells, constituting in situ a median rod, when the mesoblast plates are cut off 

 laterally, and the neurochord is defined above. Such a derivation has much in its favour, 

 if we consider such sections as are given in PL III. figs. 2 and 11, and it is the conclusion 

 adopted by Balfour and Deighton. In the case of the chick they found a median plate 

 of cells, not as yet divided into mesoblast or hypoblast, together with a short column of 

 cells originating from the primitive streak (No. 19, p. 186), and these form the notochord. 

 In Cyclostomes (Petromyzon) the notochord is formed by a vertical reduplication of axial 

 hypoblast-cells, as Calberla (No. 39) showed, and as Balfour confirmed (vide No. 11, 

 p. 87, figs. 39, 40) ; but whether this holds true for the Elasmobranchs, or whether axial- 

 layer cells, as above stated for the chick, form it, Balfour found himself unable to decide. 

 This uncertainty in regard to the origin of the notochord is further shown by the fact 

 that Rudwaner was of opinion that it arose from the epiblast ; while Kingsley and Conn 

 considered it hypoblastic, as also did Calberla for Petromyzon, Syngnathus, and Rana. 

 Braun, again, held that in the parrots the notochord was mesoblastic. 



In Teleosteans Kupffer affirms the origin of the notochord to be one of the unsolved 

 problems of embryology, and he declines to come to a decision on the question (No. 87, 

 p. 222). We have pointed out, however, that its hypoblastic origin is most in accordance 

 with the sections. The large cells (nc, PI. IV. fig. 56) above the primitive enteron, 

 there is little doubt, are the first traces of the notochord, which further forward is already 

 partially defined. Fig. 11, PL III., again, is most satisfactorily interpreted as demon- 

 strating the meeting of cells from above (the neurochordal proliferation) with the noto- 

 chordal cells (hypoblastic proliferation) below. The cells of this longitudinal rod, nc, present 

 much the same features as the adjacent cells, mes and hyp, though the neurochordal cells, 

 ne, above always exhibit a more or less depressed appearance. At its anterior end the 

 notochord grows rapidly forward, and, as Scott found in Petromyzon, it extends beyond 

 the hypoblast of the alimentary canal into the cephalic region. There is, in fact, an 

 anterior proliferation of notochordal cells (No. 146, p. 145). We see that in a series of 



