EMBRYOLOGY OF THE SEA BASS. 
217 
: ])eriblastic wall [p. w.), which as yet contains bnt a single circle of nuclei. There takes 
' place at about this time a flow of protoplasm from the wall towards the center of the 
1 blastoderm, whereby the central periblast layer becomes thicker than in earlier stages. 
During the next hour there is an active multiplication of nuclei in the periblast 
! wall. The indirect method of division is followed, and the karyokinetic figures ai’e 
very conspicuous, especially in surface views (Fig. 24). Succeeding the single ring of 
I nuclei shown in Fig. 23 there is a stage with two concentric rings, though the two 
’ rings can not be distinguished at all parts of the periphery. After this the arrange- 
l' ment of the nuclei is no longer a regular one, as is proved both by surface views and 
[| series of sections. In Fig. 28 the periblastic wall contains four nuclei, but other parts 
I of the wall show only two or three (Fig. 27). All the periblastic nuclei 6f a blasto- 
. : derm do not suffer division at the same time, but it sometimes happens that the nuclei 
|i of a particular region all divide at once, as shown iu Fig. 24, though on the opposite 
side of the blastoderm from which this view was taken the nuclei were in general at 
rest. 
The multiplication of the nuclei in the wall is followed by a gradual migration 
towards the center of the blastodisc. In Figs. 27 and 28 the inner nuclei have begun 
to creep in this direction, and the migration continues until nuclei are scattered all 
through the periblastic layer. This migration from the edge of the blastodisc is illus 
j trated by the series of sections, Figs. 29, 30, and 31, which are from blastoderms of 9.1 
1 hours, 14 hours, and 10 hours, respectively. The periblastic wall, which is so prominent 
at the beginning of migration (Fig. 29), grows less conspicuous towards its end (Fig. 
j 31). This is due in part to a gradual How of the periblastic protoplasm towards the 
|| center of the blastodisc, and in i)art to the increasing size of the blastodisc, the edge 
: of which, as it grows round the yolk, carries with it the periblastic wall. - 
I The histological chaiige which occurs in the periblastic nuclei, and which appears 
I to be universal in Teleosts and Selachians, comes on iu the early stages of migration. 
j| After the stage shown in Figs. 28 and 29, of about the same age, there is no longer any 
I indirect division of nuclei ; and the nuclei themselves, which have hitherto not differed 
i in appearance from those of the blastodermic cells, now become greatly vacuolated and 
1 - also flattened. They gradually increase iu size and their contour becomes very irregu- 
j' lar, owing to the development of protuberances. The peculiar character which they 
I retain throughout embryouic life is fully acquired before invagination begins (Fig. 32, 
j' PI. xci, and Fig. 40, PI. xciii). 
The physiological use of the periblast nuclei and protoplasm is not known. The 
I suggestion has, however, been advanced (Hoffmann, 17 ; Ziegler, 47) that the nuclei have 
( the function of working over the yolk into some shape which is easily assimilated by 
j the blastodisc cells. Their uniform histological character in the Teleosts and Selachians 
; naturally leads one to believe they have some special physiological function, the more 
so because, though ancestrally a part of the entoderm, iu the ontogeny (of the Teleosts, 
at least) they take no share iu forming the embryo. Like Hoffmann (17), Wencke- 
bach (43), Ziegler (47), Heuneguy (18), and iu opposition to Kupffer (26), and Gensch 
(13), 1 find that the nuclei do not give rise to blood cells, nor do they contribute to the 
formation of the alimentary canal. The ultimate fate of the nuclei is associated with 
the final disappearance of the yolk sac through the agency of the liver. 
