DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES, 713 
significance; but OELLACHER, Donrrz, Ryper, and others agree that it is merely an arti- 
ficial product, and due to the action of reagents. It is difficult to accept the latter view, 
after the careful observations of VAN Bamprxkr, who admits that in the trout and carp 
it is absent, as seems to be also the case in a large number of Teleosteans at St Andrews; 
yet since a cavity of this nature, remarkable for its deep situation and transient nature, 
has been seen in other blastoderms (e.g., Aves and Ganoids), it may justifiably be 
regarded as a normal structure, and perhaps due rather to the exigencies of the cleavage- 
process than to ancestral causes. If, as Warrman holds (No. 159, p. 296), “the case of 
Ascidia (Kowalewsky), of Sycandra (Schultze), of Anodonta and Unio (Flemming), of 
Clepsine and Euawes, and numerous cases like the latter, show that the blastoccel arises 
by the cells being pushed asunder in the process of cleavage,” then the segmentation- 
cavity when it is present can have no profound ancestral meaning, such as Van Bam- 
BEKE urges ; but is of interest merely in connection with modifications in the ovum, by 
which the area embraced in segmentation is greatly reduced. This reduction impli- 
cates a mechanical difficulty, resulting in the formation of a chamber, which is appro- 
priately named a segmentation-cavity or blastoccel. Probably every instance of a 
blastoccel may be explained in this manner, and it may thus co-exist along with the 
germinal cavity. The former, it is generally admitted, becomes obliterated, whereas the 
latter persists, and must be regarded as the remnant of the primitive enteron. Its 
persistence in the embryo is of importance, for it is an essential point in the gastrula 
that “it should directly or indirectly give rise to the archenteron” (No. 10, p. 457). 
That in forms so various as Gallus, Rana, Acipenser (No. 82), and Balanoglossus 
the segmentation-cavity is transient, and has no relation to the blastopore, is proof 
that it cannot be regarded as enteric, for the archenteron has always relation to the 
blastopore. In speaking of the cavity in the Teleostean ovum as germinal, we merely 
do so to distinguish it from the segmentation-cavity (blastoceel), which is wholly 
another structure, though the name does not necessarily imply any ulterior meaning. 
Nor is this course discordant with the conclusions of Teleostean embryologists in general; 
for OELLACHER distinctly affirms that the germinal cavity produced by the lifting up*of 
the germinal mass is the sole cavity observed by him in Salmo fario, and he failed to 
find a central segmentation-cavity, as was the case also with Van Bampeke in the ova 
of this species, and of Cyprinus; and Kuen, though he speaks of a segmentation- 
cavity, formed by the lifting up of the blastoderm, really means the germinal cavity 
(No. 79, p. 197, and pl. xvii. figs. 11 and 12), this latter cavity being also recognised 
by Rreneck (No. 137, p. 356), Grrr, Henneauy, Owssannikow, and Wem. JANosik 
observed a cavity in the germ, and an earlier one between the yolk and the lower layer 
cells, and he termed the former “ segmentation-cavity.’* It is not a little curious that 
Ryper, while holding that the germinal disc of Teleosteans is equivalent to the entire 
Amphibian ovum, yet regards the cavity outside the dise (germinal cavity) in the former 
as homologous with the deeply placed chamber (segmentation-cavity) in Rana and the 
* Archiv f. Mikr. Anat., vol. xxiv. 
VOL. XXXV. PART III. (NO. 19). 5Y 
