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 32-cell stage in T. 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 (PI. II. fig. 9, n), 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. Balfour 

 states that they increase by division (No. 10, p. 109), and nuclei frequently show a 

 transverse line coinciding with the short diameter (PI. IX. 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 disc in Teleostei is similar to that in Elasmo- 

 branchs, as Balfour 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. 



Oellacher, who refers more especially to the nuclear zone as described by Kupffer, 

 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. 



t Lankester 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). 



