DEVELOPMENT AND LIFE-HISTOEIES 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 (PL II. 

 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 Eansom 

 seems to have observed (No. 24, p. 466). 



The primary segmentation-nucleus is rarely visible in the germinal disc,* though 

 Kupffer noted it as a clear homogeneous vesicle, fifteen or twenty minutes after fertilisation, 

 situated in the basal stratum of the blastodisc of Clupea (No. 87, p. 206). In the section of 

 the blastoderm of Gadus ceglefinus, at the 5th hour, when two blastomeres are completed, 

 we see that the nucleus (n) occupies a position slightly above the basal stratum, and presents 

 surrounding radial structures, apparently prolongations of the nuclear substance itself 

 (PL II. 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 blastodisc, 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 Eansom failed to make them out (No. 1 27, p. 467) ; but, when not diaphanous, the nuclei 

 may appear, e.g., in the 2-cell stage of Gastrosteus 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 (PL 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 blastodisc, within an hour or more from the first appearance of the disc, and 

 extends outwards, its course being preceded by puckerings, as though the two masses 

 were drawing apart, and producing the beaded structure described by Balfour 

 (No. 11, p. 391). The diverging course of the cleavage-plane is not opposed to the 

 " loi 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 Lereboullet found the nucleus in Perca with 

 difficulty, whereas in Esox it was well seen (No. 93, p. 513). 



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



