252 BULLETIN OF THE UNITED STATES PISH COMMISSION. 



Fig. 245. Part of transverse section, showing multiple karyokinesis and formation of nests of nuclei. 

 Stage like that shown in fig. 252. Part of section behind and to one side of invagination 

 area. Same series as fig. 241. c n, cell nest at surface; y n, cell nest in yolk; y n ] , cell in 

 multiple karyokinesis, situated in yolk hall. 457 times natural size. 



Fig. 246. Part of transverse section through embryo in invagination stage, in, area of invagination. 

 211 times natural size. 



Fig. 247. Part of section of egg to show nest of nuclei at surface. 211 times natural size. 



Fig. 248. 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. 



Fig. 249. Part of section of egg containing two nuclei, this one near surface. 285 times natural size. 



Platk 53. 



Fig. 250. Surface view of embryo in 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 iine drawn through 

 any proliferating center. Numerous grannies, 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. 



Fig. 251. 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. 



Fig. 252. 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- 

 ating cells; e a, embryonic area; In, area of invagination; y n, cell nest, produced by 

 multiple karyokinesis. 89 times natural size. 



Fig. 253. Part of longitudinal section of intestine of embryo in a late stage of development, showing 

 concretions in the lumen of the organ, bm, basement membrane; ep, intestinal epithe- 

 lium; }'■> intestinal concretion. 360 times natural size. 



Fig. 254. 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. 



Fig. 255. 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 ; e c, ectoderm ; Mes-ent, mesendoderm ; In, pit of invagination ; OD, 

 optic disc. 89 times natural size. 



Fig. 256. 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. 



