484 _ PROCEEDINGS OF SECTION D. 
tion, and what slight observations I have been able to make on 
this stage may be reserved till | have had the opportunity of a 
more thorough study. As in some other teleosteans, the germinal 
disc is formed as a result of impregnation, and an unimpregnated 
ovum presents no trace of such a structure. 
The Blastodisc and the Process of Segmentation. 
The blastodisc makes its appearance a little on one side (the 
upper) of the future ectodermal pole of the egg. When fully 
formed, and before segmentation has commenced (a phase which, 
judging from its rarity in preserved specimens, must be of very 
brief duration), it is a small circular disc, around which is 
gathered a narrow zone of periblastic protoplasm. From the 
periblastic zone there radiate outwards a number of branching 
protoplasmic threads, which soon become lost in the yolk and in 
the thin investing layer of periblast. The plane of the first 
cleavage is vertical to the surface of the blastodisc, and is 
inclined at an angle to the plane passing through the long axis 
of the egg. In the next stage, of which many specimens were 
obtained, there are four symmetrically-arranged blastomeres 
forming a quadrilateral blastodisc with rounded angles. The 
next change brings about a disturbance of the symmetry, for two 
of the four cells subdivide in such a way as to give rise at once 
to the appearance of a long anda short axis in the blastodise, 
which now consists of three pairs of cells, arranged right and left 
on either side of a median line—the future long axis of the 
embryo. Of these, the middle pair are larger than the others, 
and each of them very soon becomes divided into two by a 
transverse fissure. Thus is reached the stage of eight cells, in 
which four pairs of cells are arranged symmetrically on either side 
of the middle line. During those phases of segmentation the 
blastoderm has undergone some increase in size, probably at the 
expense of the periblastic material, which has become much less 
evident, the radiating threads having disappeared altogether 
shortly after the beginning of segmentation. 
The next stages are marked by the considerable increase in 
thickness of the blastoderm, which soon projects prominently 
from the surface of the egg, and by the appearance of a ring of 
marginal cells differing to a marked extent from those of the 
remainder of the blastoderm. This ring first appears in the 32- 
cell stage, when it consists of ten rather narrow cells encircling 
the remainder. At first it is on a level with the rest of the 
blastodisc, but while the latter bulges more and more the marginal 
cells remain nearly ona level with the surface of the vitellus, 
eventually becoming tucked in beneath the steep edge of the 
central part of the blastodisc. 
