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



the thickened carina, ne, which presses upon the yolk (vide PL IV. fig 4). Very often 

 it is so distinctly separated from the epidermal cells above, that a fissure intervenes, 

 forming at the sides quite a spacious interstice (PI. III. fig. 2). The cells of the neurochord 

 are full and rounded (ne, PL IV. fig. 4), but as downward proliferation proceeds, those 

 forming its lateral boundary become columnar, and unmistakably mark off the neuro- 

 chord from the adjacent cells, especially in the fore part of the trunk. In this region the 

 cells so rapidly proliferate ventrally and laterally, that they come into direct contact with 

 the limiting hypoblast, hy, below and at the sides, or at most permit a mere trace of 

 mesoblast to find a place there. Further back (PL IV. figs. 5a and 56) the mesoblastic 

 plates, mes, lie upon each side, and its ventral ridge alone touches the hypoblast, hy, while 

 above it is limited only by the flattened stratum of epiblast, ep. Both layers of epiblast 

 seem to extend over the blastoderm, and form the outer stratum of the yolk-sac, while 

 below lies the extended hypoblast, which rests directly upon the periblastic cortex of the 

 yolk. It is below the second epiblastic layer, which here assumes the character of a loose 

 mucosa, a rete Malpighii — or rather in the lowest stratrum of this mucous layer, that the 

 pigment occurs as amorphous bodies which extend over the surface of the yolk. In 

 P. platessa and other forms, in which the epiblast lies immediately upon the periblast, 

 the hypoblast being apparently absent, the pigment may send processes into the yolk- 

 cortex ; indeed pigment may develop in the periblast itself as described on a subsequent 

 page. 



Notochord. — In the earliest sections of the trunk, no trace of the notochord is seen, 

 the neurochord, ne, being limited below by the single layer of hypoblast, hy, and having 

 the thick mesoblast, mes, upon each side (PL IV. figs. 5a-5d). About the time that the 

 lip of the blastopore has reached the equator, a median mass of cells (nc, PL III. fig. 11) 

 intervenes between the keel of the neurochord, nee, and the hypoblastic stratum, hyp. 

 These (notochordal) cells are rounded, and rapidly show a somewhat concentric 

 arrangement, quite unlike the depressed cells of the stratified neurochord above 

 (PL III. fig. 11; PL IV. fig. 56). The notochordal cells, nc, it is true, are not 

 separated by any definite line of demarcation from the ventral ridge of the neurochord, 

 ne ; but as the cells of the latter are unmistakably squeezed upwards by the pressure 

 of the notochord below, this could hardly happen were the cells of the notochord a 

 downward proliferation of neurochordal cells. The ventral ridge of the neurochord is 

 evidently indented and its cells greatly flattened by these axial cells below. In such a 

 section of the early notochord as shown in PL III. fig. 11, the possibility remains that this 

 axial rod of cells is a remnant of median mesoblast, left when the lateral mesoblastic plates 

 are sundered as protovertebraj, but the difficulty of such derivation lies in the fact that 

 the mesoblast never appears to be confluent in this region ; on the contrary, when once 

 the notochord is indicated, it is sharply marked off from the mesoblast on either side. 

 Thus in the section (PL IV. fig. 10), while the notochordal mass, nc, is not clearly 

 separated from the hypoblast, hy, below, or from the epiblast (neurochord), ne, above, a 

 very distinct line of division passes between it and the lateral mesoblastic plates, mes, 



