DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 707 



Around the cleaving nucleus a radial disposition of granules is seen,* the centres of 

 the radii being the nuclear apices, for the nucleus itself becomes biconical and shows 

 longitudinal striae prior to the division which soon takes place across its middle or shorter 

 axis, this transverse separation being followed by the division of the surrounding blas- 

 tomere. The process indeed accords perfectly with Balfour's account of the Elas- 

 mobranch ovum (No. 15, pp. 394-5). Occasionally a blastomere is seen to contain 

 two distinct nuclei, illustrating indeed the stage of the process figured in PL II. fig. 2, 

 a stage which Lereboullet also clearly observed, for he says — " Dans un de ces lobes j'ai 

 trouve une cellule qui avait deux noyeaux distincts rapproches l'un de l'autre " 

 (No. 93, p. 484), and Balfour similarly speaks of a double nuclear condition (No. 11, 

 p. 396). Though usually very distinct and centrally situated, the nucleus sometimes 

 becomes diaphanous, and appears to be absent. Such an enuclear condition is hardly 

 possible, though Professor Kay Lankester, it is true, speaking of the blastoderm of 

 Cephalopods, says — " I have most fully satisfied myself that temporarily many of the 

 segmentation-products are devoid of nucleus" (No. 90, p. 39); and Lereboullet, when 

 noting the fact that all through cleavage each blastomere contains a nuclear body, adds 

 that "often it may be absent" (No. 93, p. 484); while Bambeke could find no trace 

 of nuclei in Leuciscus rutilus, but accounts for it by the similar refrangibility of the 

 nucleus and the matrix in which it is situated (No. 20a). This disappearance of the nuclei 

 is not an uncommon phenomenon in cell-division. Very often (PI. II. fig. 1) a body 

 apparently of the nature of a clear vesicle occupies the place of the deeply-stained nucleus 

 in sections, or such a vesicle occurs only partly occupied by a nuclear remnant (PI. II. 

 fig. 1). These unstained bodies were noticed by Balfour, and he felt uncertain whether 

 they were nuclei imperfectly stained, or nuclei in course of being formed (No. 11, p. 395). 

 In the living egg the phenomena of segmentation are followed without much difficulty, 

 especially in pelagic forms. The two primary cleavage-planes are seen to cut each other 

 at right angles ; but the third cleft is parallel to the second (PL X. fig. 4). On the 

 completion of the third cleft the blastoderm consists of six cells, of which the central 

 pair are larger than the others. At this stage the blastoderm is rudely rectangular, an 

 outline altered by the next cleft, which passes once more parallel to the second and third 

 clefts, through the large central cells (PL XIV. fig. 8). The size of the blasto- 

 meres is far from uniform after the 8-cell stage. The 16-cell stage is completed by a 

 separate furrow traversing each cell and bisecting it, so that the total number of 

 blastomeres is thus doubled at about the fourth or, it may be, the sixth hour after 

 fertilisation. It would appear that in the Teleostean ovum, as also in the fowl and 

 Selachian, the two primary furrows alone are really regular, the succeeding furrows being 

 in varying degree irregular, so that the blastomeres are not seen to increase with the 



* Oellacher observed the concentration of yolk-spherules round one or two centres in the segmentation-spheres, 

 but this is not the phenomenon he described, though Balfour understood Oellacher to refer to the behaviour of the 

 ordinary nuclei during segmentation. Ryder also speaks of numerous fine granules aggregated round two centres in 

 the first cleavage-stage. 



