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 Whitman 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, i.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, 11, 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 (PI. IV. figs. 56 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 (PI. 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 ujyper 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, 11 ; 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," Oellacher, " Umhiillungshaut," Reichert, 

 " Deckschicht," Gotte); while the under stratum, which always consists of more than one 

 layer of rounded cells, is the sensory epiblast, 11 (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 PI. II. 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 Ryder'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 



* Lerkboullet also in his forms (Perca and Esox) made out his epidermoidal layer only when the equator was 

 reached (No. 93, p. 493). 



