216 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
connected with the yolk (or periblastic) protoplasm. The marginal cells so connected 
eventually lose their cell outlines, and are drawn into the surrounding protoplasm. 
In Ctenolabrus there are two concentric rings of cells thus made use of. Before the 
apiiearance of their paper it had been held by Kupff'er and others that the nuclei 
originate in the yolk independently of the blastodisc nuclei. Hoffmann. (17), on the 
other hand, claimed that the first segmentation nucleus underwent a horizontal cleav- 
age, the upper daughter nucleus giving rise to the nuclei of the blastodisc, while, 
from the lower daughter uucleus were derived the nuclei of the periblast. Agassiz 
and Whitman have sufficiently criticised Hottmaun’s account and figures, and while 
there is good reason to believe that the periblast nuclei do not originate in all Teleosts 
from the peripheral cells of the blastodisc (von Kowalewski, 27) their criticism seems 
to me a very just one. 
I have found that the nuclei develop in Serranus in a manner almost identical 
with that in Ctenolabrus There are minor differences concerning the number of rings 
of marginal cells drawn into the periblast, etc., but my account is essentially a con- 
firmation of that of Agassiz and Whitman. 
At the end of segmentation the marginal cells of the blastodisc are flattened and 
do not take the stain as readily as the other cells. In a surface view (Fig. 21, PI. xc), 
they ap[)ear as a wreath of pale cells round the periphery of the blastoderm. This 
wreathof cells, often observed (Kupffer, 24 ; Van Beneden, 40 ; Eyder,35; Cunningham, 
8; Henneguy, 18), has been and is still the subject of great misconstruction. Kupffer 
and others believed that these cells were formed round nuclei, which had originated in 
the yolk, and that they then passed out of the yolk and were added to the blastodisc. 
It might be thought that the very lucid and exact account of Agassiz and Whitman 
would have cleared this iiart of teleostean embryology from any shade of uncertainty. 
But in his last paper (18) so old a student of the Teleosts as Henneguy concludes 
that the cells of the “wreath” are passing out of the periblast “pour s’ajouter an 
germe” (p. 461). 
In Fig. 21 the marginal cells, though very different in appearance from the rest of 
the blastoderm, still retain their cell outlines. They are even marked off from the sur- 
rounding periblastic protoplasm, which continues to form round the edge of the blasto- 
derm the “ early periblastic ridge.” A few minutes later there is no longer any line 
of separation to be seen between these cells and the outlying protoplasm, though they 
are still marked off from one another (Fig. 22, PI. xc). Sections through this stage are 
the most important for the study of the formation of the periblast. Fig. 25, PI. xci, 
is such a section, in which the right half presents a slightly older condition than the 
left. On each side the marginal cells pass without the interruption of the ridge into 
the cortical layer of protoplasm {cor.p.). On the left, however, the marginal cell still 
preserves its earlier shape; the segmentation cavity' cuts its way into it (compare ear- 
lier sections. Figs. 20, 19, 18, etc.). But on the right the cell is flatter, and the whole 
body passes uninterruptedly into the central periblast layer (c. p.). The section shows 
plainly enough that the marginal cells are being drawn into the periblastic protoplasm. 
An hour later (Fig. 23, PI. xc) the marginal cells have fused with one another, and 
the lost cell outlines are now only indicated by the accumulations of protoplasm 
round the nuclei. In sections through this stage it is seen how completely the periph- 
eral cells have lost their identity in the process of fusion with each other and the 
periblast. In Pig. 26 the marginal cells of Pig. 25 have been metamorphosed into the! 
