24 RALPH E. WAGER, 



cleavage the cleavage cavity is established (Fig. 24, PI, IV,), 

 After the third, the cleavages take place in such an irregular 

 fashion as to make it impossible to follow them. As a result, 

 however, of further divisions, there is formed a hollow mass of 

 cells which in some cases vary but little in size, while other sec- 

 tions show a much greater variation (Figs, 26 and 27, PI. IV,). 

 This increases in size by the continued tangential division of the 

 cells until a very large blastula is formed (Fig. 27, PI. IV.), The 

 inner ends of the cells contain the larger portion of the pseudo- 

 cells while the nuclei are located near the outer part. 



Embryonic Layers. 



The cells of the blastula above described are nearly of the same 

 size, though some project into the cleavage cavity farther than 

 others. Division of these longer cells occurs and the entoderm 

 is thus formed by a process of delamination. With these daughter 

 cells are carried the larger part of the pseudo-cells, so that when 

 the process of entoderm formation is finished the ectoderm cells 

 are comparatively free from them. By the continued division of 

 the primitive ectodermal cells in this fashion the cleavage cavity 

 becomes almost entirely obliterated. The entoderm cells also 

 divide and the solid mass of cells is formed. That the ectoderm 

 cells may themselves be crowded into the cleavage cavity as ob- 

 served by Brauer('9i«) found no confirmation in the preparations 

 examined. Radial divisions were the most numerous with few 

 tangential divisions. 



No uniformity in the size of the eggs produced was observed 

 for the amount of material available for the growth of the egg 

 determines in large part its ultimate size. 



The completion of the formation of the entoderm marks the 

 beginning of the processes resulting in the embryonic membranes, 

 which, as in other species oi Hydra, consist of an ©uter chitinous 

 and densely spiculate shell and a very thin inner membrane just 

 beneath it. The former takes its origin as follows : The ecto- 

 dermal cells constituting the outer layer of the embryo send out 

 slender processes as shown in Fig. 13, PI. II., and Fig. 29, PI, 

 IV, These are most frequently formed in groups of two or more 

 and are remarkably large when compared with the size of the 



