DEVELOPMENT AND LIFE-HISTOEIES OF TELEOSTEAN FISHES. 713 



significance; but Oellacher, Donitz, Ryder, 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 Bambeke, 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 Whitman holds (No. 159, p. 296), "the case of 

 Ascidia (Kowalewsky), of Sycandra (Schultze), of Anodonta and Unio (Flemming), of 

 Clepsine and Euaxes, and numerous cases like the latter, show that the blastocoel 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 blastocoel. Probably every instance of a 

 blastocoel 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 G alius, 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 (blastocoel), 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 Sahno fario, and he failed to 

 find a central segmentation-cavity, as was the case also with Van Bambeke in the ova 

 of this species, and of Cyprinus; and Klein, 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 pi. xvii. figs. 11 and 12), this latter cavity being also recognised 

 by Rieneck (No. 137, p. 356), Gotte, Henneguy, Owsjannikow, and Weil. 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 

 Ryder, while holding that the germinal disc of Teleosteans is equivalent to the entire 

 Amphibian ovum, yet regards the cavity outside the disc (germinal cavity) in the former 

 as homologous with the deeply placed chamber (segmentation-cavity) in Rana and the 



* Archivf. Mikr. Anat, vol. xxiv. 

 VOL. XXXV. PART III. (NO. 19). 5 Y 



