OP ARTS AND SCIENCES. 55 



this half, together with the adjoining periblast, constitutes the ento- 

 dermic hemisphere of the ovum. The difference of constitution be- 

 tween the marginal and the central cells, which is brought out by a 

 differential staining, is manifest in sections, though not so decided as 

 in later stages. 



In passing to the o2-cell stage, the central portion of the blastodisc 

 becomes two cells deep, each of the four cells «, b, a, d (Fig. 2) 

 splitting horizontally, as shown by the vertical position of the amphi- 

 asters. The marginal cells divide obliquely, so that the outer half of 

 each still remains in continuity with the periblast. 



One hour after the 1 6-cell stage, we find the blastodisc three cells 

 deep in the central portion ; nearer the margin, two cells deep ; and 

 at the margin, one cell deep, as shown in Fig. 3. 



At this time we find that the marginal cells, which are continu- 

 ous at their outer and inner angles with the periblast, are still more 

 strongly characterized than in the 1 6-cell stage. Their relations with 

 the periblast are essentially the same. The central cells show the 

 brownish tint characteristic of osmic acid staining, while the marginal 

 cells are much lighter. In preparations stained with Grenacher's 

 borax-carmine, the central cells take little or no carmine ; but the 

 marginal cells and the periblast stain well, thus bringing out a very 

 decided contrast in color. This contrast in color extends to the nuclei, 

 those of the central region being deeply browned, those of the marginal 

 cells being stained red. The radial arrangement of the protoplasm 

 around the nuclei is much more strongly accentuated in the marginal 

 than in the central cells. 



If the osmic acid is stronger, or is allowed to work longer than 

 usual before transferring to the chrom-platiniim solution, the contrast 

 is often very much strengthened. In some of the mounted prepara- 

 tions, the central cells are very deep brown, almost black, while the 

 marginal cells are light yellowish brown. 



In most of the mounted preparations of this stage, the marginal 

 cells appear to be well marked off from the periblast, although con- 

 tinuous with it ; but we have one in which some of these cells have 

 already entered the syncytial condition, which usually appears about 

 two hours later. In this case they are considerably flatter than the 

 marginal cell seen at the left in Fig. 3. But the syncytial condition 

 is not yet fully established, coming and going as often as the cells 

 divide. 



Two hours after the 1 6-cell stage, the blastodisc is from two to four 

 cells deep ; and one or two marginal cells (in section) present the 



