DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 745 
sections (Pl. IV. figs. 5b—5d) through the posterior region during the early stages of the 
notochord ne, 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 
(ne, Pl. 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 (ve, Pl. IL. 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, ne, 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 (ne, Pl. IV. fig. 10), save at its 
extreme aboral end (and Batrour 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 (PI. 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, &c.). The process in Clupea, according 
to KuprFEr, is not such as we have described for our forms, for the refractive dises, he 
states, are formed by the confluence of minute granular particles in the primary cells as 
* Phil. Trans., 1882, ii. p. 365, 
