DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 723 
and entoderm (Hy), the cells of both being very much flattened; but along the embryonic 
axis several layers are present, and the cells are, in the living germ, more rounded and 
fuller than elsewhere. Similar larger cells also occur at the margin (Pl. IV. figs. 5/, 
and 7), and to the presence of these, as well as their closer arrangement, no less than the 
greater number of cells, is due the thickened appearance of the marginal belt or rim (br). 
It is clear that the blastoderm covers a very large superficial area, when compared with 
its extent at the close of segmentation, and this extension is largely, as we have hinted, 
a process of “ flattening out” undergone by the originally rounded or polygonal cells of 
the archiblast. The cells are thus expanded superficially; but doubtless there is also a 
marginal addition of cells—periblastic in origin. 
Beneath the rim and embryonic axis a single layer of cells intervenes, separating the 
germ from the yolk. This layer is, in fact, the third primary layer or hypoblast 
(Darmdrusenblatt), and its mode of origin is a point of great interest. How does it 
arise? The answer to this question is by no means easy, but the view that it is 
invaginated, z.e., an inflection of the epidermal layer, is grounded upon appearances in 
the living ovum, and prepared sections (PI. Il. figs. 15, 17, hy) no less than upon 
phylogenetic considerations. A folding-in of the epiblast is indeed seen at a very early 
stage, but, when the germ has thinned out, this involution is more apparent (PI. II. 
figs. 10, 17), and the centripetal advance of the rim can be readily followed by continuous 
watching, for, starting as a narrow peripheral band very slightly denser than the rest of 
the blastoderm, it advances slowly towards the central point of the animal pole. 
This region, known as the embryonic scutum (OELLAcHER’s Embryonalschild), coincides 
with the embryonic radial thickening, which, as already noticed, is present from a very 
early stage. LEREBOULLET calls it the “ bandelette primitive” or ‘“ germe embryonnaire,” 
as being in his view the first indication of the embryo (No. 94, p. 255), but this is not 
so, the thickened radius preceding by an interval of many hours the inflection of the 
hypoblast, and being already distinguishable, when the germinal cavity appears. At 
first the scutum is a mere tubercle in the Salmonoids, as LEREBOULLET says, though 
flatter and more tongue-like in Gadoids, which pushes out from the rim and progresses 
towards the pole opposite to the blastopore. As it advances and extends laterally, it 
brings visibly into prominence the embryonic thickening, which, however, already exists, 
and when the blastoderm covers about one-fifth of the vitellus, this hypoblastic layer 
spreads out asa scutiform film or membrane beneath the embryo. That this process is one 
of true invagination is disputed. Goérrn, Hennecuy, CunnincHaM, KinasLey, Cony, and 
others hold that it is so; whereas OxLLACHER regards it merely as a delamination, 
a simple differentiation in situ of the deepest layer of the primary entoderm, and this 
view RypeEr and others adopt. Kuprrer, Van Bampeke, His, Kuery, and G. Brook 
regard the sub-blastodermic protoplasm or periblast as the source of this layer. 
LEREBOULLET speaks of it as a vegetation or proliferation (No. 94, p. 253), though he also 
seems to resort to a kind of mechanical transference of cells (No. 93, p. 488). We know 
that in Elasmobranchs this layer is formed partly by conversion of lower layer cells 
