DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 721 
and due, there can be no doubt, to planes of cleavage passing as linear depressions from the 
margin of the dise outwards. No cells could actually be seen to be completely segmented 
and added to the margin of the disc, nor could this be ascertained by study of the living 
ovum, for such cells transferred into the germ would enter the lowest stratum of the disc, 
and would therefore pass beneath the margin along the basal region—this margino-basal 
portion of the blastoderm being especially unfavourable for study in the living condition. 
There is no evidence against Broox’s view, that matter passes into the archiblast in the 
early stages, and thus nourishes it—a view similar to that of Horrman (No. 68), viz., 
that the nucleated periblast performs the function of provisional blood. 
VII. Empryonic SHIELD anD Rim. 
We have traced the development of the ovum up to the stage which immediately 
precedes the formation of a distinct embryonic trunk, coincident with the radial 
thickening of the blastoderm. No clear differentiation into layers can as yet be 
made out, though the upper stratum is usually distinguished as a layer of ectoderm 
(OetLacHER’s “hornblatt”) or epiblast (Pl. Il. figs. 1-3, 6, and 15, ep)—the cells 
below, which form the main mass of the germ, being endodermal or lower layer 
cells (//). ‘This saucer-shaped blastoderm (PI. II. fig. 19), consisting of two germinal 
leaves or layers, arches over the germinal cavity, while peripherally it is in contact 
with the cortical protoplasm of the yolk, chiefly that part of the cortex distinguished 
as periblast. Then commences epiboly, that remarkable process which RaTHKe, in 
1832 (No. 129), was the first to describe in Teleosteans. The germinal matter which 
originally clothed the vitelline globe as a film, and afterwards becomes segregated 
at the animal pole, is now seen apparently retrogressing, and again encloses the yolk, 
not as a homogeneous envelope, however, but as a segmented cellular blastoderm. 
With the commencement of the process the blastoderm flattens (PI. II. fig. 15, bdm), 
and the vertical height of the germinal cavity (gc) is by this depression so much 
reduced as to form a mere fissure, though otherwise its relations remain unaltered. On 
the second or third day, in the Gadoids and other forms here referred to, this flatten- 
ing is clearly shown; and LereBouLer, who describes it in Hsox, says that during the 
first half of the second day the blastodermic vesicle (7.e., the germ) flattens more and 
more, its two opposing walls touch,* and it becomes moulded as a serous envelope round 
that part of the egg which it covers like a watch-glass (No. 93, p. 488). By this process 
of flattening and extension meridionally over the yolk-ball, the germ becomes distinctly 
thinner. This decrease in thickness is especially noticeable, LEREBOULLET says, in Salmo 
fario as compared with Perca, and epibolic extension in the trout is much less rapid 
than in the latter. Variations, too, occur in pelagic ova, but these are doubtless caused 
* If our interpretation of LEREBOULLET be correct, it is not accurate to speak of the two layers, viz., the thin germ 
and the periblast, as really touching, though the interspace becomes less and less. 
VOL, XXXV. PART III. (NO. 19). 5 Zz 
