DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 741 



though in another section of the same date (PL III. fig. 13) the notochord and mesoblast 

 are not distinctly separated.* In the chick the early notochord is continuous laterally 

 both with the mesoblast and hypoblast (No. 19, p. 185) ; while Van Bambeke, in agree- 

 ment with Oellacher, decides from his sections that the notochord is directly mesoblastic 

 (No. 114 ; also see his fig. 14, pi. iv.). A comparison of a large number of sections shows 

 that the mesoblast, mes, is very clearly separated as two lateral plates, e.g., as in PL IV. 

 fig. 5a ; but the notochord, even when detached from the hypoblast, and apparently in 

 intimate connection with the epiblast (neurochord), is never united to the myotomes. 

 The hypoblast, hy, it is noteworthy, is hardly distinguishable in this region, as though it 

 had been almost entirely used up in the formation of the notochord, for at the sides it is 

 well-defined. Balfour noticed a similar thinning out of the hypoblast, and he states 

 that only by high powers could the continuity of the stratum be made out (No. 14, 

 p. 683). Van Bambeke again denies that the hypoblast exists here at all, affirming that 

 the notochord is at first in direct contact with the periblast below (No. 20a, fig. 15, pi. 

 ii.), a layer of cells being afterwards pushed in from each side, and thus separating the 

 notochord from the cortex of the yolk. The character of the cells, on close examination, 

 shows the distinguishing features insisted on earlier, viz., the (dorso-ventrally) depressed 

 condition of the neurochordal cells, ne, and their arched stratified disposition ; whereas 

 those of the notochord do not exhibit these features, and the contrast is still more 

 emphatic at a later stage. In addition to their rotund condition, the notochordal cells 

 are seen in longitudinal section to have a transverse arrangement, such as would be pro- 

 duced by an antero-posterior pressure (nc, PL IV. fig. 15), and this is interesting as indi- 

 cating, what we have already suggested was possible (see p. 729), viz., that the notochord 

 may be pushed forward to a certain extent from the primitive streak. 



Unlike the condition in Elasmobranchs, the notochord of Teleosteans is at first clearly 

 differentiated in the mid-trunk or mesenteric region (PL XXII. fig. 12, nc), and gradually 

 extends forward, ending indefinitely above the middle of the cardiac rudiment, as in Molva 

 vulgaris, on the first or second day (PL V. fig. 8). It curves downward, and sometimes 

 seems to turn slightly to the left, as in T. gurnardus, on the ninth day. A section through 

 the otocystic region (PL IV. fig. 4) shows a mere trace of median hypoblastic proliferation, 

 while in the post-mesenteric region the activity of the hypoblastic cells has resulted in the 

 formation, not of a distinct notochord, but of an arch of columnar enteric cells bridging 

 over a cavity (PL IV. fig. 56), suggesting a condition identical with that represented in 

 Balfour's figure of this region in Petromyzon (No. 11, fig. 39, p. 86), in an Elasmobranch 

 (No. 15, fig. 1, c, pi. xxix.), and in Lacerta (No. 14, figs. 2, 3, pi. xix.) ; while Scott and 

 Osborn's figure of Triton (No. 147, fig. 5, pi. xx.) no less closely resembles it. The 

 last-named observers clearly saw that the notochord originated from the upper wall 

 of the alimentary canal, as is indicated in their figure just mentioned. The outgrowth of 

 the notochord from this enteric roof, figured in PL IV. fig. 5b, is not actually seen, but 



* In Lepidosteus, Balfour and Parker noticed a similar sharp separation from the mesoblastic plates, while the 

 hypoblast had more intimate relation to the notochord, but they could not decide as to its real origin. 



