404 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



U11011 the optic disks, the bases of the appendages, and other parts of the embryo. They also 

 pass to the extra-embryonic surface of the egg. In Fig. 91 one of these wandering cells (Y. C.) 

 is ajiproaching the surface, while another (Y. C) nearly touches it; and in PI. xxix, Fig. 34, 

 which represents a portion of a section from a later series (Fig. 106) greatly enlarged, we lind 

 two yolk elements (Y. C.) quite at the surface. They are triangular in outline, oue of the flat- 

 tened sides being applied to the surface of the egg. Histologically their nuclei are more gran- 

 ular and stain with less intensity than the nucleus of the ordinary epiblastic cell (Ep.), which 

 appears spindle-shaped in vertical section. But between a cell like that seen in Fig. 91 (Y. C), 

 where the long axis of the nucleus is at right angles to the surface, or cells like those shown in 

 Fig. 34, where the nucleus is dattened against it and the ordinary epiblastic oell, a variety of tran- 

 sitional phases can be found. TLis is most clearly illustrated in the next stage. 



The egg already described (Fig. 93) shows some important changes. The structure of the optic 

 disks and ventral plate is readily seen in the transverse (Figs. 92, 94, 95) and lateral longitu- 

 dinal sections (Figs. 96, 97). The optic disk, which in stage lir consisted of a single stratum of cells, 

 (Fig. 69), is now a thick cell-mass two-thirds the size of the ventral plate. In it we still distin- 

 "■iiish a small area of cells with large granular nuclei (Fig. 93, C. M.), which, as we see in Figs. 96, 97, 

 C. M., O. D., is clearb' differentiated. It occupies a position just without (or external with respect 

 to the longitudinal median axis) the center of the disk. The nuclei of surrounding cells are not 

 more than half their size. These large cells do not all lie at the surface, but form a solid mass 

 extending iuto.the yolk. The evidence of karyokinetic figures shows that these cells are dividing, 

 and usually in planes perpendicular to the surface. This results in the crowding of the cells, and 

 also in their migration from the surface to parts below it. 



In Fig. 90 there is a cell (ec.) whose nucleus has sunk below the level of the surrounding cells, 

 but the cell protoplasm still reaches uiJ to the surface. Such cases render one cautious in pro- 

 nouncing iiositively upon the emigration of cells, but sections like that given in Fig. 94, and the 

 fact that cell divisions seem to be for the most part in one plane, convince me that the thickening 

 is partly, if not largely, due to this cause. In Fig. 95, a large cell, the polar star of which is Just 

 below the surface, is delaniinating. Cases of this kind were rarely noticed, but were observed at 

 a later stage (PI. xxxix, Fig. 102, ec). That wandering cells attach themselves to the optic disk, 

 there is little doubt. They can be traced in all stages of progress from the region of the ventral 

 idate to the neighborhood of the disk (Fig. 90, Y. C.) until they fltially come in contact with it. 



The thickening of the optic disk described in Stage in (PI. xxxiii, Fig. 69), is therefore efl'ected: 

 (1) partly, perhaps largely, by emigration of cells from the surface; (2) partly by delamiuation ; 

 (3) by the accession of wandering cells; (4) by the indirect cell division of the elements constitut- 

 ing the deeper part of the disk. 



Chromatin grains make their appearance somewhat abruptly at this stage (Fig. 96, S.'-.) and 

 they serve to explain in some degree the peculiar granular nature of nuclei in earlier stages. They 

 originate by the degeneration of cells in the ventral plate and in other jjarts of the embryo, and 

 probably correspond to what Keichenbach has called in the crayfish " secondary mesoderm cells." 

 Their history has been fully traced and will be discussed in Section vi. 



The structure of the ventral plate (Fig. 92) resembles that of an earlier stage (Fig. 85) with 

 the difference that the nuclei are considerably larger and contain from one to several large s[)ore- 

 like masses of chromatin. This cell mass has not inci'eased in bulk at a rate corresponding with 

 the growth of other parts, since elements are being continually subtracted from it and added to 

 the yolk. This loss on the part of the ventral plate is made good not only by cell divisions, 

 but also by continued emigration from the surface (Fig. 9S, Ab.). 



An embryo six hours older than the last is represented by three longitudinal sections (Figs. 

 98-100). The optic disks resemble in size and general shape the three pairs of rudimentary limbs. 

 The abdominal fold is not yet formed and the labrum is undeveloped. The stomod:eum (Fig. 38, 

 Std.) is jsfit making its appearance as a slight invagination of epiblast on the middle line between 

 the first pair of anteuuic. The number of chromatin balls (S) and degenerating cells (S. C.) has 

 greatly increased. We see them in all parts of the embryo. Sporelike nucleoli are seen in cells 

 of the ei)iblast and are confined to no part of the embryo, but they are most characteristic of the 

 ventral plate and optic disks. 



