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 striz 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 Baurour’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 PI. II. fig. 2, 
a stage which LEREBOULLET also clearly observed, for he says—‘‘ Dans un de ces lobes j'ai 
trouvé une cellule qui avait deux noyeaux distincts rapprochés l'un de lautre” 
(No. 93, p. 484), and Baxrour 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 Ray 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 LereBouLLer, 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 Bampeke 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 (Pl. IL. 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. I. 
fig. 1). These unstained bodies were noticed by Batrour, 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 OrLLacuer to refer to the behaviour of the 
ordinary nuclei during segmentation. Ryper also speaks of numerous fine granules aggregated round two centres in 
the first cleavage-stage. 
