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 disc 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 Brook's view, that matter passes into the archibkst in the 

 early stages, and thus nourishes it — a view similar to that of Hoffman (No. 68), viz., 

 that the nucleated periblast performs the function of provisional blood. 



VII. Embryonic 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 

 (Oellacher's "hornblatt") or epiblast (PL II. figs. 1-3, 6, and 15, ep) — the cells 

 below, which form the main mass of the germ, being endodermal or lower layer 

 cells (11). This saucer-shaped blastoderm (PL 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 (PL II. fig. 15, hdm), 

 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 Lereboullet, who describes it in Esox, says that during the 

 first half of the second day the blastodermic vesicle (i.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 Z 



