726 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
CUNNINGHAM’s suggestion may indeed precisely express the fact, when he hints that 
this layer may be produced in Salmonoids by delamination, and in the Gadoids and other 
forms by a centripetal process (No. 48). 
In either case the final result is the establishment of a continuous layer of flattened 
cells, which extends underneath the blastoderm, and forms an alar expansion on each 
side of the trunk of the embryo. AGassiz and Wurman speak of it as three or four 
cells deep below the embryonic axis; but this is true only for a slightly later stage, after. 
proliferation has commenced. A typical section of the Teleostean on the establishment 
of the hypoblast, z.e., when the yolk is about half covered, shows (as in Pl. II. fig. 17) a 
single-layered corneous epiblast, ep, formed of fusiform or flattened cells, which roofs over 
a thick mass of cells for the most part derived from a second layer of epiblast, the 
sensory or neurodermal stratum, //, and lastly, the single layer of cells composed of the 
invaginated hypoblast, hy. The more or less acuminate snout of the embryo often appears 
to dip into the hypoblast in front, or rather the hypoblast (hy) seems to creep up and 
overlap the anterior end of the embryonic carina, car. (PI. III. figs. 5 and 6). Posteriorly 
the hypoblast does not exhibit the flattened or squamous character, but forms a small 
tract of full, conical or cubical cells, hy (Pl. IV. figs. 5b and 6). These cells, which are 
quite at the blastoporic termination of the embryo, arch over a horizontal cavity, and 
form indeed a superior enteric roof, constituting, as CUNNINGHAM strongly and ably urged, 
a plate of dorsal hypoblast, and giving origin, as will be shown, to the notochord. 
These two important points fall to be considered shortly. 
The germinal area after completion of cleavage may be said to present three successive 
phases,—first, it*is composed of archiblast cells (Pl. II. figs. 1 and 2) of fairly uniform 
size, polygonal, uninucleate as a rule, and formed of clear protoplasm free from yolk- 
spherules ; secondly, an upper stratum becomes slightly flattened, and may be dis- 
tinguished as ectoderm, ep (PI. II. fig. 3), while the mass of unaltered cells below forms 
the “lower layer” or primitive entoderm, //; thirdly, the ectoderm, though at first a 
single layer, subsequently exhibits three or four layers, and the outer stratum is the 
epidermal or corneous epiblast (“ Hornblatt,” OrLtacuer, “ Umhiillungshaut,” RercHert, 
“ Deckschicht,” GOrrr); while the under stratum, which always consists of more than one 
layer of rounded cells, is the sensory epiblast, JJ (Sinnesblatt of OELLACHER), and this 
latter layer by rapid proliferation forms the neurochordal carina, constituting the main 
mass of the embryonic thickening, which below is limited by the single hypoblastic stratum, 
hyp. These three stages are represented in Pl. IL. figs. 2 and 3. 
Epiblast.—Little can be added by way of special remark in regard to this layer. 
Certainly the late distinct differentiation of the epiblast in Teleosteans forms a point of 
contrast to the condition in Elasmobranchs and Amphibians; but RypeEr’s statement 
that the epiblast, with the other germ-layers, is only split off when the shield appears 
(No. 141, p. 494),* will not apply to the forms mainly treated of here, for the epiblast is 
* LEREBOULLET also in his forms (Perca and Esox) made out his epidermoidal layer only when the equator was 
reached (No, 93, p. 493). 
