252 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Fig. 245. 
Fig. 246. 
Fig. 247. 
Fig. 248. 
Fig. 249. 
Fig. 250. 
Fig. 251. 
Fig. 252. 
Fig. 253. 
Fig. 254. 
Fig. 255. 
Fig. 256. 
Part of transverse section, showing multiple karyokinesis and formation of nests of nuclei. 
Stage like that shown in tig. 252. Part of section behind and to one side of invagination 
area. Same series as fig. 241. cn, cell nest at surface; yn, cell nest in yolk; yn', cell in 
multiple karyokinesis, situated in yolk ball. 457 times natural size. 
Part of transverse section through embryo in invagination stage, in, area of invagination. 
211 times natural size. 
Part of section of egg to show nest of nuclei at surface. 211 times natural size. 
From section through embryo in invagination stage, showing multiple karyokinesis and 
formation of nuclear nests at surface, like that shown in fig. 247. 211 times natural size. 
Part of section of egg containing two nuclei, this one near surface. 285 times natural size. 
Plate 53. 
Surface view of embryo m invagination stage. The embryonic area of this egg lies in front 
of the shallow pit. Cells are most numerous immediately in front of this depression and 
about the extreme anterior margins of the exposed surface. The rapid proliferation of 
cells in these regions gives rise on the one hand to the thoracic-abdominal plate, and to 
the optic disks on the other. Karyokinetic figures of dividing Cells are seen scattered 
over the entire surface of the egg. The equatorial plate is in each case vertical, and may 
make any angle with the longitudinal axis of the embryo, or with a line drawn through 
any proliferating center. Numerous granules, the products of cell degeneration, are 
commonly seen. The reverse side of this egg shows nothing peculiar. Nuclei are there 
less numerous, and the superficial cells are larger. Flecks or clouds of granules, floating 
in the yolk below the surface, are seen here and there with no regularity. The nuclei 
over most of the surface, excepting those at the extreme periphery, were drawn by aid of 
the camera lucida, as were the cell outlines in the more central parts. From a picro- 
sulphuric acid preparation, stained in borax carmine, the egg cut in two, and the 
hemispheres mounted in balsam. August 6, 1892. 500 times natural size. 
Plate 54. 
Part of transverse section through area of invagination (in), showing columnar surface cells 
(ec) filled with yolk and invaginate cells, lying between and within yolk masses. Some 
of the latter cells just below the surface are undergoing degeneration; some in the 
deepest parts of the egg are creeping with their long pseudopodia between the yolk 
spherules. These play the part of phagocytes and also contribute to the tissues of 
the embryo. 360 times natural size. 
Surface view of embryo in region of invaginate area, showing clusters of cells at surface, 
produced by multiple karyokinesis. A, anterior; P, posterior end of egg; Deg, degener- 
atiur cells; e a, embryonic area; In, area of invagination; y n, cell nest, produced by 
multiple karyokinesis. 89 times natural size. 
Part of longitudinal section of intestine of embryo in a late stage of development, showing 
concretions in the lumen of the organ, b m, basement membrane; e p, intestinal epithe- 
lium; p, intestinal concretion. 360 times natural size. 
Part of transverst section through invaginate area of an earlier stage than last, showing 
in- wandering masses of cells. A, anterior; P, posterior; In, pit of invagination; y c, 
invaginate cells. 89 times natural size. 
Part of longitudinal section through area of invagination, showing the advancing cumuli 
of cells, which are pressing into the deeper parts of the egg and investing large masses of 
yolk. The distinctly columnar superficial cells, gorged with yolk, are also shown. In the 
embryonic area these become very tall, and beneath them there are clouds of disorganized 
chromatin granules, the remains of degenerated cells. A, anterior; P, posterior; Deg, 
degenerating cells; ec, ectoderm; Mes-ent, mesendoderm; In, pit of invagination; 01), 
optic disc. 89 times natural size. 
Concretion from intestine of an embryo which Was nearly ready to hatch. Teased from 
picro-sulphuric acid preparation and mounted in glycerin. 360 times natural size. 
