THE AMERICAN LOBSTER. 205 



About 110 cells are present iu the egg shown in fig. 223, and not far from 220 in 

 the next phase (fig. 221). The lack of uniformity in cell division which was present 

 iu 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. 

 221 and 225 is represented in fig. 211, 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 uuextruded egg. The nucleus was spherical and lay in a spherical 

 island of granular protoplasm. The nuclear membrane was very delicate, and could 

 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 iu 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. 219) the rays being exceedingly numerous and delicate, reaching far 

 out among the yolk-spheres. In an egg which was cut into 56 sections, the first two 

 cells appeared iu the twenty-fourth and thirtieth sections, respectively. In each case 

 the nucleus was spherical, and 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 01 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, where 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 and 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. 21 ( J and 220; a section of 

 the egg is represented in fig. 212.) 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 



