GASTRULATION. 



69 



separate in one of its daughter cells, and the nucleus in the other about to undergo 

 amitotic division; fig. 64 K, a nucleus similar to the last occurs in a cell high up in 

 the blastoderm, and near it a cell which has undergone amitosis; fig. 64 M and N, 

 two cells which are undergoing an extraordinary type of division; they contain 

 many asters, conspicuous centrosomes, but no chromosomes; in N the cell is sub- 

 dividing into three daughter cells. 



The significance of these phenomena is commented upon on a later page, in 

 the discussion of megaspheres and yolk nuclei in their relation to germ layers. 



To resume the question of gastrulation : 



The third stage in the writer's material is represented in surface view in plate v, 

 fig. 31. It differs from the earlier stage shown in this way, plate v, fig. 30, in the 

 following regards: (i) The circumgerminal ring, which had gradually been 

 extending and carrying its nuclei peripherad, has faded out over the surface of the 

 yolk, its proximal zone now alone noteworthy. (2) There is a conspicuous antero- 

 posterior differentiation. The region of the blastopore is indicated by a short 

 transverse shadow, marking the cavity of the archenteron, and the segmentation 

 cavity is denoted by a broad transverse area, the ends of which as they approach 

 the rim of the blastoderm bend backward, giving a somewhat crescentic shape. 

 Three sections of this blastoderm are figured, the first, fig. 65, is sagittal, the 

 second, fig. 65 A, passes between the points A- A, shown in the surface view, the 

 third, fig. 65 B, between the points B-B. Comparing the sagittal section fig. 65, 

 with that of the earlier stage, fig. 64, we observe increased growth at the posterior 

 rim of the blastoderm; the germinal wall instead of shelving forward, now shelves 

 backward, especially near the surface of the egg, still having below a sharp 

 shoulder* against which lies the remains of the posterior lip of the blastopore, pm< 

 of earlier stages, this region, in short, is being overgrown by the blastoderm as it 

 progresses hindward. The archenteron thus remains, as at a, separated only 

 imperfectly from the segmentation cavity, sc, which is now of great size. This 

 condition, indeed, is well shown in the more lateral sections, figs. 65 A and B, and 

 they indicate as well the narrow limits of the archenteron; the sides of which, it 

 will here be seen, are practically confluent with the sides of the segmentation cavity. 

 As in the preceding stage, noteworthy relations exist between the blastoderm and 

 the yolk. We observe, for example, that in the more lateral section a tongue of 

 coarse yolk passes inward close to the surface of the germinal wall, and we obtain 

 evidence that the row of neighboring cells has been formed by actual outbudding. 

 In these cells amitosis occurs, as in the previous stage. These cells, it may be 

 remarked, do not long remain in their subjacent position, for, identified by the 

 coarse yolk they contain, they can be traced into the blastoderm and are found 

 widely scattered among other cells. 



* I'is-ti-i'fs is a second shoulder which corresponds to the anterior germinal wall of the stage shown in fig. 63. In 

 both regions, then, the blastoderm has overgrown the surface of the egg. 



