DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 709 
of the cavity (germinal cavity). Soon after also the larger cells fall off, and we now get 
complete analogy with the Amphibian egg, viz., above the cavity the sensory layer 
composed of smaller cells, and below the large cells for the body of the embryo.* This is 
not the case, however, in osseous fishes, for on the completion of segmentation, an 
epiblastic layer can barely be distinguished: it is not by any means well marked.t 
Germinal Cavity.—With the completion of segmentation the blastoderm undergoes 
a change of the most striking character. It lengthens out (Pl. I. fig. 17) and soon 
becomes elevated from the yolk, so that a chamber ge. (Pl. II. fig. 15, a-d), not 
coincident with the centre of the disc, is formed between its under surface and the 
vitellus (y) below.t Hitherto the whole of the inferior face of the blastoderm has rested 
immediately upon the yolk (y) (see Pl. Il. figs. 1-8) or rather upon a portion of the 
yolk-cortex ; but now the inner surface being raised it rests only by its periphery, and 
the eccentrically situated cavity intervenes between it and the vitelline mass. In 
Trigla gurnardus the sub-blastodermic cavity is plainly visible on the second day, 
when the germ covers barely a third of the surface of the yolk. 
A cavity has been observed in some Teleostean ova at a much earlier stage; but it is 
probably a precocious dehiscence and of minor significance. Such a cavity in the 
gurnard may be formed even before the first cleavage is accomplished, and is probably 
due to the cleavage-process, as we find to be the case in Amphioxus at the 4-cell 
stage. Acassiz and Wurman found a similar cavity in Ctenolabrus at the 16-cell: 
stage, while His describes one at the 8-cell stage.§ Such cavities, of a transitory 
nature, have been noticed in very many ova; in Acipenser sturio, for example, at 
the 6- to 8-cell stage, according to KowaLEwsky, OwsJANNIKow, and WAGNER ; 
while Rauber saw it in the Avian ovum at the 4-cell stage (No. 132, p. 6). The last 
named observer distinctly affirms that the early cavity he saw is not the homologue of 
the later embryonic chamber, generally distinguished as the “ Keimhéhle ;”|| and as this 
is a point of no little importance, it is desirable to dwell upon the distinction here 
implied. The very existence of a cavity, either “segmentation” or “germinal,” has 
been denied by some investigators. It has been pronounced by Donrrz amongst others 
(No. 52, p. 600) to be merely an artificial product ; and Kuprrer suggests something of 
the same kind, though unwilling to lay stress upon his results, which were negative 
(No. 87, pp. 214-16). That the somewhat complex methods now adopted in 
laboratory work are calculated to produce occasionally artificial changes in embryonic 
* Rreneck also considered that the embryo originated in one point of the peripheral thickening which occurs at 
the point of contact between the yolk and the germ. 
+ Goetre affirms that there is no distinct differentiation of any of the germinal layers in the multicelled condition 
of the dise if we except the outer “ epithelial” (Archiv f. Mikr. Anat., iv., 1868). 
t Rreneck, op. cit., observed the central part of the germ lifted off its underlying part. 
g It is this cleavage-cavity which Ryper considers as probably homologous with the cavity of the false amnion 
(Amer. Nat., xix., 1885, and Jour. Roy. Mier. Soc., Feb. 1886, p. 45). 
|| This later cavity BaLrour, in common with most observers, names the segmentation-cavity, though he says it 
is not a well-defined chamber, and remarks that “it may even be doubted whether a true segmentation cavity ... . 
is present.” 
