DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 719 
ordinary vertebrate tissues has a tendency to stir up a like condition in surrounding parts. 
So early as the 82-cell stage in 7. gurnardus, numerous nuclei, precisely like those after- 
wards present in the periblast, were observed, irregularly scattered beneath the blastoderm. 
Some of these nuclei, which were in close proximity to each other, coalesced and formed 
large irregular structures. 
On one occasion careful focussing brought out beneath the cells of the blastoderm (in 
an ovum of the species just referred to, of which the yolk was about half enveloped) the 
faint outlines of periblastic nuclei, while, in an oblique view of the invaginated rim its 
under surface was somewhat regularly nodulated by the nuclear projections which thus 
protrude into it from below (PI. II. fig. 5). 
The blastoderm of Gastrosteus spinachia at a certain stage shows, scattered through- 
out its extent (Pl. II. fig. 9, x), large bright nuclei, often showing many nucleoli. These 
nuclei, as suggested elsewhere (No. 124, p. 493), are probably periblastic, and they 
persist for some time after the closure of the blastopore. 
After their appearance close to the margin of the disc, they extend outwards, while 
at the same time they also pass inwards, and form a nucleated stratum beneath the 
blastoderm. They progress centripetally, and eventually stud the periblast-floor of the 
germinal cavity, and are visible through the roof formed by the translucent blastoderm ; 
but whether they increase by cleavage or spontaneous endogeny is not clear. Barour 
states that they increase by division (No. 10, p. 109), and nuclei frequently show a 
transverse line coinciding with the short diameter (Pl. LX. fig. 10), but the further 
constriction and “ direct” division of an example of such nuclei into two daughter-nuclei 
was not made out,* and it is probably true that they arise and multiply precisely like the 
nuclei named ‘“‘autoplasts” by Professor LankesTER in the ovum of Cephalopods—arising 
and multiplying not by cleavage, but originating de novo as independent segregations.t 
The behaviour of the nuclei outside the dise in Teleostei is similar to that in Elasmo- 
branchs, as Baurour clearly states that whatever influence the nucleus may have in 
ordinary cases of cell-division, it may yet undergo precisely similar changes without 
exerting any influence on the surrounding protoplasm. In Elasmobranchs the nuclei of 
the disc are rounded and regular in form, while those in the yolk are irregular in shape, 
and provided with knob-like processes. The cone-like nuclei are only found in the earlier 
stages, and they possess no distinct membrane. 
OxELLACHER, who refers more especially to the nuclear zone as described by Kuprrer, 
says there isno need to resort to free-cell formation, inasmuch as its protoplasm is the 
same as the rest of the archiblast, hence, in each, the segmentation-process is the same. 
BaMBEKE ingeniously suggests that an endogenously-formed yolk-nucleus may give 
origin to these nuclei, and that the cells of which they are the centres are segmented 
more slowly than the cells of the disc (No. 20a, p. 4); but, as previously noted, 
* The failure to observe “direct” division will not, of course, appear strange to those who accept karyokinesis or 
indirect division as the sole process of nuclear multiplication, but all visible forms of division are here included. 
+ LAaNKESTER is also of opinion that the cells of the perimorula in Gammarus locusta arise as isolated structures 
like the autoplasts of Cephalopods (No. 92, p. 63). 
