THE AMERICAN LOBSTER. 
205 
About 110 cells are present in tlie egg shown in fig. 223, and not far from 220 in 
the next phase (fig. 224). The lack of uniformity in cell division which rvas present 
in the earlier stages now entirely disappears. In other words, the individual rhythms 
of the component cells of the embryo appear to be in harmony. 
At the next and following divisions (fig. 225) the protoplasm approaches nearer 
and nearer the surface, and the animal and vegetative poles are no longer distin- 
guishable. A surface view of an egg intermediate between the stages shown in figs. 
224 and 225 is represented in fig. 244, plate 52. The invagination stage soon follows. 
INTERNAL CHANGES IN SEGMENTATION. 
The histological changes which take place in development up to the beginning of 
the invagination stage will now be considered. 
The segmentation nucleus in a single egg, from a batch which I obtained on 
August 1, was very eccentric in position, and in appearance resembled the germinal 
vesicle of the unextruded egg. The nucleus was spherical and lay in a spherical 
island of granular protoplasm. The nuclear membrane was very delicate, and conld 
hardly be demonstrated in sections, while the chromatin had a rather coarse granular 
appearance. 
The first division takes place near the center of the egg, and the products move 
away from each other. In two eggs examined, each of which contained two nuclei 
(possibly the pronuclei), one nucleus in each case lay nearer the center and the other 
nearer the surface. The nuclei are relatively small, and after a few divisions become 
very much smaller. Each is surrounded by a rayed body of protoplasm, in some 
cases (as in fig. 249) the rays being exceedingly numerous and delicate, reaching far 
out among the yolk-spheres. In an egg which was cut into 5G sections, the first two 
cells appeared in the twenty-fourth and thirtieth sections, respectively. In each case 
the nucleus was spherical, aud the cell protoplasm formed a compact oval island, giving- 
off no long characteristic pseudopodia, as are seen in fig. 238. In another case, where 
the egg was cut into 64 sections, one cell appeared in the thirty-first and its sister 
cell in the thirty-ninth section of the series, the latter being in process of division. 
In the second and third segmentations which follow, producing four and eight 
cells, the products separate and migrate toward the surface. The greater number 
tend to move toward the side of the egg corresponding to the animal pole, Avliere the 
yolk is first segmented (figs. 215-218, plate 50). 
In an egg containing nineteen cells, with yolk undivided, eight were in various 
stages of karyokinesis. Some cells were nearer, others farther from the surface, the 
majority being about midway between the center aud periphery, in different parts of 
the egg. 
In another egg, where the segmentation of the superficial yolk was completed, 
just thirty cells or yolk pyramids were present. (See figs. 219 and 220; a section of 
the egg is represented in fig. 242.) The constrictions of the yolk are not simply 
superficial, but cleavage planes often reach halfway down to the center of the egg. 
The nucleus with its rayed protoplasm lies toward the center of the convex face of 
each segment, but is still separated from the surface of the egg by a considerable layer 
of yolk. The entire protoplasm is thus distributed among the yolk segments, none of 
it remaining in the undivided yolk mass. In surface views the nuclei can be seen 
shining through the thin stratum of yolk which lies between them and the surface. 
Sometimes a segment is partly overgrown by the surrounding cells and squeezed below 
