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.! 



Germinal Cavity. — With the completion of segmentation the blastoderm undergoes 

 a change of the most striking character. It lengthens out (PI. II. fig. 17) and soon 

 becomes elevated from the yolk, so that a chamber gc. (PI. II. fig. 15, a-d), not 

 coincident with the centre of the disc, is formed between its under surface and the 

 vitellus (y) below.| Hitherto the whole of the inferior face of the blastoderm has rested 

 immediately upon the yolk (y) (see PI. II. figs. 1-3) 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. Agassiz and Whitman 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 Kauber 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 " Keimhohle ; "|| 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 Donitz amongst others 

 (No. 52, p. 600) to be merely an artificial product ; and Ktjpffer 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 



* Rieneck 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. 



t Goette affirms that there is no distinct differentiation of any of the germinal layers in the multicelled condition 

 of the disc if we except the outer " epithelial " (Archivf. Mikr. Anat., iv., 1868). 



J Rieneck, op. cit., observed the central part of the germ lifted off its underlying part. 



§ It is this cleavage-cavity which Ryder considers as probably homologous with the cavity of the false amnion 

 (Amer. Nat., xix., 1885, and Jour. Roy. Micr. Soc, Feb. 1886, p. 45). 



| This later cavity Balfour, 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." 



