EMBRYOLOGY OF STOMOTOCA APICATA 
343 
The migration of the entoderm continues for some hours, and 
finally the blastocoele becomes solidly filled with this newly 
developed tissue. At first the cells are crowded together without 
any definite arrangement except that due to pressure. Then those 
cells that are situated next to the ectodermal layer change shape, 
becoming columnar and assuming the appearance of a more or 
less distinct layer Such an arrangement is shown in fig. 22. Later 
a separation of the cells takes place in the center of the entodermal 
mass. This is the first beginning of the coelenteric cavity, which 
gradually increases in size; and finally the entodermal cells be- 
come arranged in a single layer around this cavity. 
DIFFERENTIATION OF THE ECTODERMAL CELLS 
When the larva is about twenty-four hours old and about the 
same time that the entodermal tissue begins to arrange itself into 
the definite inner germ layer, a differentiation begins in the 
ectodermal tissue. The interstitial cells now make their appear- 
ance here and there by crowding in between the bases of the co- 
lumnar ectodermal cells. These latter cells which heretofore were 
straight, cylindrical structures with their sides parallel to each 
other, now become more irregular; some assume conical forms, 
others spindle shapes according to the pressure of the neighboring 
cells. Also, about this time, or a little later, small oval refractive 
bodies make their appearance usually in the interstitial cells, 
occasionally in the entodermal cells also. These small ovoid 
structures gradually push their way toward the exterior, and 
finally come to be situated in or between the ectodermal cells at 
the surface. They are developed into nematocysts. 
ATTACHMENT 
When the larva is about forty-eight to fifty hours old it settles 
to the bottom, loses its cilia and thus its power of movement is 
lost. It is now ready to become attached. The method of attach- 
ment in Stomotoca differs from that usually described and re- 
garded as typical for the hydroid larva; in which case they settle 
