DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 717 
dise being the first to appear, the rest following in succession (No. 88, p. 217). At the 
12th hour, in the gurnard, the nuclear zone forms a conspicuous spotted belt round the 
disc, and the yolk in certain views seems to be distinctly pitted by them (PI. II. fig. 5). 
A little later they are less distinct. When the blastoderm has extended over one-third 
of the yolk-surface, traces of the nuclei are still to be seen (PI. XIV. fig. 7, np). Thus, 
at the 25th hour, in the gurnard the blastoderm is surrounded by a continuous belt 
of protoplasm, beyond which few or no granules exist. Those previously seen have been 
overlapped by it, but are visible underneath towards the rim. In the surrounding 
protoplasm no large nuclei appear, and only a few of the granules of the previous stage. 
Often, during the early period of the nuclear zone, the nuclei appear in groups as if 
multiplying by division, this being well marked in the ova of Gadus morrhua; but on 
the second day the nuclei are invisible, and only a granular ring surrounds the disc. 
How do these nuclei arise? Three possible geneses are suggested,—they may be 
derived from the nuclei of the blastoderm, as ScHuLrze, OELLACHER, WHITMAN, and 
WeNcKEBACH (No. 158)* hold ; or they originate directly or indirectly from a primary yolk- 
nucleus (Horrman, E. van BreNnepen); or lastly, they may be endogenously formed as 
independent segregations of active protoplasmic particles (Kuprrer), either from the 
marginal cells, or from the cells which fall from the lower surface of the “ segmenta- 
tion-cavity,” or rather germinal cavity, and which fuse with the periblast. WENCKEBACH 
asserts that no nuclei or cells arise either in the periblast or in the yolk, and that the 
nuclei of the periblast, after their separation from the blastoderm, degenerate and take no 
part in the formation of the embryo.t The appearance of the extra-embryonic nuclei later 
than the nuclei of the germ—further, their first manifestation close to the margin, and 
their increase centrifugally from the blastoderm, point, it cannot be denied, to a blasto- 
dermic origin. Their derivation from an original single yolk-nucleus has not been 
demonstrated by any observations, nor does it appear to be supported by the manner in 
which the nuclei become visible, though it accords best with the theory that the multi- 
nucleate condition is less primitive than, and derived from, the uninucleate. This con- 
tention Burscuit has devised, and he adduces the case of certain Infusorians in which not 
only is the multinucleate condition prior, but actually gives rise to the uninucleate con- 
dition—many nuclei coalescing before nuclear cleavage takes place (No. 36, pp. 212-13). 
It must be observed, on the other hand, that ENGLEMANN (No. 54, pp. 576-7) and ZELLER 
(No. 161, p. 360) have shown that in Opalina the multinucleate is unmistakably derived 
from a primary uninucleate condition. The existence of a primary yolk-nucleus in 
Teleosteans still remains to be demonstrated. If, by segmentation of this nucleus, the 
periblast-nuclei are produced, appearances in the living ovum afford little evidence of 
it; but if the nucleus dissipates, and later, becomes aggregated again at numerous 
superficial centres, then this view is not without support. 
Kurrrer, Kier, and other authors regard the nuclei we are considering as free 
* Ryper recently adheres to this view (U. S. Com. Report for 1885 (1887), p. 490). 
+ No. 158, and Jour. Roy. Micr. Soc., Feb. 1887, p. 43. 
