DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 743 



sections (PI. IV. figs, bb-bd) through the posterior region during the early stages of the 

 notochord nc, it widens out and becomes lost in the primitive streak (prs), or rather 

 merges in the upper wall of the gut, both disappearing in the caudal mass of indifferent 

 cells (prs, PL III. figs. 3 and 12), just as in Lepidosteus the notochord is not separated 

 from the lateral mesoblast, nor the latter from the neurochord, posteriorly. When ulti- 

 mately it is defined, and extends from its hind emargination to its oral termination 

 (nc, PI. V. fig. 4), its cells do not long retain their primitive condition. They are not, as 

 in Triton, primarily large cells which divide into small cells, and again break up to form 

 larger cells once more (No. 147, p. 467), but are cells of small diameter — agreeing with 

 such as for the most part compose the embryonic trunk, and become larger by an increase 

 of their substance. Thus in a haddock of the fourth day (nc, PL III. fig. 13) with the rim 

 at the equator, they can only with difficulty be distinguished from the mesoblast-cells, 

 mes, on each side; yet when the blastopore is just closing (fifth day, PL IV. fig. 10), 

 these cells, nc, are conspicuous for their large size and rounded contour, while their tend- 

 ency to assume a radial arrangement is marked. The larger size of the cells in transverse 

 section must, no doubt, in some measure be due to the forward pressure mentioned on a 

 previous page, for only three or four cells reach across the diameter of the notochord. In 

 their smaller, earlier condition six to eight cells extend across the same diameter. 



While the notochord is well defined posteriorly (nc, PL IV. fig. 10), save at its 

 extreme aboral end (and Balfour and Parker found a similar obliteration of the notochord 

 posteriorly in Lepidosteus)* anteriorly it is even more distinctly marked (nc, PL IV. fig. 

 11), though as yet no chordal membrane surrounds it. When the blastopore is closing 

 the notochord does not reach as far as the pectoral region, but on the first or second 

 day afterwards it extends quite to the point where the cardiac swelling appears (PL V. 

 fig. 8). About the time that the lenses of the developing eyes are visible the oral end 

 of the notochord is sufficiently well marked to exhibit the characteristic flexure in 

 front of the heart ; but at its aboral end it spreads out slightly, and vaguely terminates 

 in the tail which is now defined and prominent (nc, PL XXIII. fig. 9 ; PL V. figs. 8 

 and 10). Transverse striations soon cross the notochord, due to the continued for- 

 ward pressure of its cells from behind, and cells here and there are seen breaking 

 down, so that discoidal plates, or rather irregular vertical septa separating intra- 

 cellular chambers, are formed. From its oral to its aboral end a continuous series of 

 these chambers appears, resembling the " interrupted pith " of botanists (nc, PL IV. fig. 

 12). The process of vacuolation, of the breaking down, and aggregation of flattened 

 cells in serial fashion, is preceded by the assumption of a radial arrangement in the cells 

 about to suffer alteration, their nuclei showing a centripetal movement, so that they are 

 mainly found along the central line of the notochord (nc, PL IV. fig. 10, just as 

 Oellacher represents in No. 114, Taf. iv. fig. xvi 2 , &c). The process in Clupea, according 

 to Kupffer, is not such as we have described for our forms, for the refractive discs, he 

 states, are formed by the confluence of minute granular particles in the primary cells as 



* Phil. Trans., 1882, ii. p. 365. 



