DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 705 
expanded appearance. These changes of external form, which are often combined with 
an apparent dehiscence of the blastoderm and puckering of the under surface (PI. LI. 
fig. 14), are probably due to the inherent mobility of the protoplasm; but are also 
connected doubtless with the transference of the cortical matter which has not yet ceased. 
They are especially noticeable when fresh cleavage is about to commence, as RANsoM 
seems to have observed (No. 24, p. 466). 
The primary segmentation-nucleus is rarely visible in the germinal dise,* though 
Kuprrer noted it as a clear homogeneous vesicle, fifteen or twenty minutes after fertilisation, 
situated in the basal stratum of the blastodise of Clupea (No. 87, p. 206). In the section of 
the blastoderm of Gadus eylefinus, at the 5th hour, when two blastomeres are completed, 
we see that the nucleus (m) occupies a position slightly above the basal stratum,and presents 
surrounding radial structures, apparently prolongations of the nuclear substance itself 
(PL IL. fig. 18). When this nucleus has divided into two, each is seen to occupy a central 
position in the pair of newly-formed blastomeres. The two blastomeres (Pl. XXVIII. fig. 4) 
often show disparity in size, with a more or less distinct reniform outline when viewed from 
above. This disparity may be due to unequal segregation of protoplasm, or to more obscure 
causes, but the shape of the earliest blastomeres appears also to depend upon the direction 
of the first plane of cleavage; for, when this is in the shorter axis of the blastodise, the two 
cells are rudely discoidal, and are in contact by their flattened margin; or if in the longer 
direction, the result, as in the gurnard, is the production of a pair of reniform cells—the 
hilum, so to speak, of each coinciding with the proximal margin. The nucleus in each blasto- 
mere is not spherical, but slightly elliptical and flattened, showing indeed as a transparent 
almond-shaped body, when viewed in profile, and of a paler hue than the surrounding matrix. 
In the living ovum the nuclei are usually very difficult to detect during the earlier stages, 
and Ransom failed to make them out (No. 127, p. 467); but, when not diaphanous, the nuclei 
may appear, ¢.g., in the 2-cell stage of Gustrosteus spinachia and Trigla gurnardus, 
as minute irregular vesicles, like clear vacuolations distributed in each blastomere. The 
protoplasm around the central nucleus of each blastomere exhibits a radial disposition 
like the figure of the “lines of force” around a magnet (PI. II. fig. 18), but the more 
detailed features of nuclear and blastomeric cleavage are of the complex nature charac- 
teristic of karyokinesis. Each cleavage begins as a superficial indentation, which in the 
case of the first furrow commences in the centre of the straw-tinted pullulation or 
granular blastodise, within an hour or more from the first appearance of the dise, and 
extends outwards, its course being preceded by puckerings, as though the two masses 
were drawing apart, and producing the beaded structure described by Batrour 
(No. 11, p. 391). The diverging course of the cleavage-plane is not opposed to the 
“Joi centripete” of M. Serres, for the plane penetrates (centripetally) the disc. The 
vacuolations which produce the beaded appearance, while most numerous at the margin of 
* Ransom failed to make out the primary segmentation-nucleus, and indeed the blastomeric nuclei. Possibly 
various species may differ in regard to the visibility of the nuclei, for LeREBouLLer found the nucleus in Perea with 
difficulty, whereas in Hsox it was well seen (No. 93, p. 513). 
VOL. XXXV. PART. III. (NO. 19). oX 
