480 PRocEEDiyas of the national museum. 



case of this is found in the very center. Some cells show mitotic 

 figures, and in others tliere are beside the nuclei small solidl}^ staining 

 round bodies of unknown function. 



Fig. 64, derived from tig. 59, is a line example of its type. It is very 

 flat and the segmentation cavity is very much reduced. The periblast, 

 perfectly free from yolk and as distinct below as above, has a layer of 

 cells cut out of it and at the left a nucleus under the marginal cell and 

 clearly derived from it. At one point near the center the periblast is 

 reduced to a mere line. This figure, which is typical for the whole 

 blastoderm, is remarkably like His's (1898) flg. 10 for the brook trout. 



STAGE OP TWO-HUNDRED-FIFTY-SIX SX'RFACE CELLS. 



The normal type blastoderm of this stage is shown in iig. 65. The 

 cells lying near the upper surface are considerably smaller than those 

 in the lower parts nearer the periblast. To right and left are furrows 

 with dilatations helping to cut cells out of the periblast, and at the 

 center are cells nearlj^ free from it. 



Fig. 66 is plainlj^ a derivative of fig. 63 in its general outline and in 

 the reentrant angles which separate its outer periblast {o. p.) from the 

 marginal cells. The periblast is somewhat sunken in the yolk and free 

 from cells throughout the whole blastoderm. The segmentation cavity 

 is, because of this depression, large and is only partly filled with cells. 

 Neighboring sections show the upper surface to be as flat as that in 

 fig-61. 



The third type is shown in fig. 67 from a nearly central section. 

 There is a very noticeable difference in the size of the blastomeres, 

 some being fully three times as large as others. Here again are cells 

 being cut out of the basal periblast. They are in all stages from 

 rounded buds to a completely cut-out cell. Neighboring sections show 

 muclei in each of these. At the right are two cells connected by a 

 stout protoplasmic bridge. 



Fig. 68, Plate X, is a good example of the rounded type. The 

 spacious segmentation cavity is loosely filled with rounded cells. The 

 periblast is throughout the blastoderm in the form of two thick pads 

 in the '"''Rand'''' region, but in the center it is very thin and obscured 

 with yolk. Nowhere in the whole blastoderm are cells being budded 

 off' from it. In the peripheral cells there are, even in this advanced 

 stage, two cases of protoplasmic bridges. 



A nearly horizontal section through such a blastoderm as fig. 68 is 

 shown in fig. 69. This is introduced to show the arrangement of cells 

 in horizontal plane. There is here a closer aggregation of cells to 

 the periphery, the inner row being a derivative of the outer, while in 

 the center the cells are more scattering. 



Fig. TO is from a blastoderm intermediate between those from which 

 figs. 65 and 67, Plate IX, are taken. Neighboring sections are more 



