488 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



are present, the one in the periphery being- very large. This con- 

 dition is far from rare in this and later stages. Some thirty cases 

 have been particularly noted. The periblast is very thick, yolky, and 

 distinct. It contains several nuclei, and a cell is either being cut out 

 of or is in process of uniting with the periblast. In other sections 

 similar conditions are found. The reentrant angle, between the outer 

 periblast and the '■'•Rand'^ in this and tig. 78, recalls the like in figs. 63 

 and m, Plate IX, and fig. 47, Plate VIII, and in His's figures for the 

 Salmonoids referred to above. 



STAGE OF ONE THOUSAND-TWENTY-FOUR SURFACE CELLS. 



Fig. 75 represents the normal type and presents several points of 

 interest. The surface cells show a considera})le flattening, and adjacent 

 to them are other cells with their bases generally at right angles to 

 the former, making the outer layer in places two cells thick. The 

 inner cells show a tendency to run together in threes and fours. The 

 chief interest, however, centers in the periblast. This is notably free 

 from yolk and is drawn exactly as it appears. Nuclei are scattered 

 very freely throughout its entire extent in all sections, and nearly 

 surround the large vacuole to the right of the center. At the left a 

 large cell, which has recently been cut out of the '' Band,^'' is dividing 

 by mitosis. A large number of cells rest on and -mdent the periblast, 

 and are either being cut out of or added to this layer. The close jux- 

 taposition of these cells to nuclei in the periblast would seem to lead 

 to the former conclusion. 



The second type is represented in fig. 76, which, judging by the 

 number of cells in the periphery and by their size, is from a blastoderm 

 slightly younger than the preceding. The periblast is sunken deeply 

 into the yolk, and has thus nearly doubled the segmentation cavity, 

 which is sparingly filled with scattered cells. The thick periblast is 

 so obscured with yolk that no nuclei could be found. It is here free 

 from cells, but nearby sections show a condition in this respect like 

 the preceding figure. In the '■'Bechckicht,'" near the center, is a 

 binucleate cell, while its neighbor has a spindle. 



Fig. 77 is from a rounded blastoderm of about the same stage as the 

 preceding. A '' Dechschichf can hardly be spoken of here, for the 

 outer cells are nearly all round. The segmentation cavity is reduced 

 to the small interstices between the cells. The greatly thickened peri- 

 blast is full of large vacuoles, and abounds in nuclei in all the sections, 

 and near the center seems to be budding off cells. In the left '^Rand^'' 

 there is a mitotic figure fully twice as large as any in the blastomeres. 



No better illustration of the lens-shaped blastoderm so characteristic 

 of late Teleost segmentation than fig. 78 can be given. It probably 

 has been derived from a form like the preceding by the pressure of 

 the cells against the eggshell, causing the periblast to be depressed. 



