206 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
the surface, as iu fig. 238, but this must not be mistaken for one of the phagocytes or 
yolk cells, which are formed in an entirely different manner. 
Karyokinetic figures, which are clearly seen in all dividing nuclei, show that up 
to this time the plane of cell division is radial, for all cells at least which have 
approached as near the surface as those shown in fig. 242. A little later than this, fig. 
224, plate 50, and fig. 244, plate 52, when over 200 cells were present, phagocytes or 
yolk cells suddenly appear. One egg showed, when sectioned, the following condition : 
Number of cells at surface 219 
Number of yolk cells 11 
Total number of cells iu egg 230 
Number of cells in active karyokinesis : 
Radial division 15 \ ^ 
Tangential division .. 2 ) 
The yolk cells are in various stages of migration from the surface to the depths 
of the egg. Four have reached points about midway between the surface and center, 
and one of these is in the metakinetic stage of division. They originate by the 
tangential division of a certain number of the peripheral cells and by the multiplication 
of yolk cells thus formed. The peripheral cells at this time are not quite in contact 
with the surface of the egg, but are separated from it by a thin layer of yolk spherules. 
When a peripheral cell divides in a horizontal or tangential plane, the central daughter 
cell migrates into the great yolk mass, filling the egg, while the other remains at the 
surface and forms a part of the primitive blastoderm. The process is illustrated in 
figs. 236 and 243, the latter showing the formation of a yolk cell near the surface, and 
also the division of a yolk cell already formed. 
THE INVAGINATION STAGE. 
After a stage similar to that shown in fig. 225 is reached the peripheral cells continue 
to divide in radial planes, and their protoplasm soon bounds the surface of the egg. 
Cell division becomes more rapid over one side of the egg, possibly that corresponding 
to the animal pole, but this was not finally determined. An area of rapid proliferation 
is thus distinctly marked off, and in the midst an invagination of superficial cells 
occurs. This begins by the in-wandering of a few cells, which is followed by the multi- 
plication of those thus immersed in the common food stock, and by the sinking in of a 
small area of the blastoderm about this point. In an ovate egg, like that shown in 
fig. 227, the invaginate area lies toward one of the poles. 
The depression is at first very shallow, but increases considerably in depth and 
becomes a well-defined circular p>it. Later it elongates transversely (cut 26), and in 
abnormal cases a deep gutter is formed. The character of this depression is indeed 
subject to great variation. The pit at the surface lasts from four to five days, when, 
after becomiug elongated into a slightly triangular slit, by the ingrowth of the sides, 
it completely disappears. 
In an egg iu which there is a distinct patch of cells marking the area of invagina- 
tion, but where the depression is very slight or has entirely disappeared, the embry- 
onic area which lies iu front of the point of invagination is marked by a wonderful 
activity among the superficial cells. This is illustrated in fig. 252, plate 54. About 
the point of invagination there is a mass of several hundred cells, from which migra- 
tion into the yolk has taken place. Many of the cells, both at and below the surface, 
