198 HUBERT DANA GOOD ALE 



anterior part of the archenteric roof is formed of large yolk cells, 

 which become smaller posteriorly, although their granules remain 

 large. Close to the lip, granules of intermediate size are found. 

 Not far below the blastopore, the ventral mesoderm, vm, fig. 33, 

 can be seen among the yolk cells which are now almost entirely 

 invaginated. 



Finally, just before the embryo appears, the formation of the 

 archenteron is complete (fig. 34, cf. fig. 26). The ectoderm 

 cells are smaller and arranged in a single layer except around the 

 blastopore. The triangular mass of ventral mesoderm, vm, lies 

 just below^ the blastopore. The columnar yolk cells in the pos- 

 terior part of the roof of the archenteron, are clearly notochordal. 

 Anteriorly, they have not yet differentiated. The cells marked 7n 

 in the dorsal wall near the blastopore are probably mesoderm. 



Fig. 35 is a lateral section of the same egg, passing through 

 the end of the transverse blastopore. The posterior part of the 

 roof of the archenteron has differentiated into the endoderm, 

 en and mesoderm, m, the last fusing at the blastopore with the 

 ectoderm. Near the middle of the roof, mesoderm and endo- 

 derm join the still incompletely differentiated cells of the anterior 

 half. 



The yolk granules of the dorsal ectoderm cells (fig. 34), tying 

 between x and x', are of the smallest size. Between x' and b, 

 as well as in the dorsal lip between x and x" are the intermediate 

 granules, while the large ones are found in the rest of the egg. 



Mechanics of gastrulation. I shall not attempt to go into this 

 question extensively, but will merely state the conclusions to 

 which my observations and experiments lead me. Since the 3'olk- 

 cells can be seen actively migrating at certain stages and since it 

 can be shown that the cells of the equatorial band divide and 

 migrate, I conclude that the yolk cells and the cells of the Rand- 

 zone are in a condition of active movement. Neither kind moves 

 the other passively, but each is actively moving in coordination 

 with the other to its proper place. I cannot accept the view that 

 gastrulation is due primarily to increased cell-division in certain 

 places, or to increased growth of local areas of cells which cause 

 other areas to invaginate. The cause of gastrulation is an in- 



