LIFE-HISTORY OF CRASPEDACUSTA RYDKKI. 



437 



At the same time the cells at the place of invagination divide and 

 migrate into the segmentation cavity (Fig. 5). From the figure 

 it may be noted that the cells extend into the cavity in the form 

 of strands. This division and migration continues until the 

 segmentation cavity is filled with cells (Fig. 6). The cells of 

 the outer ectodermal layer are still elongate but less so at the 



FIGS. 7, 8, AND 9. 7 shows the beginning of elongation and the formation of 

 the gastrovascular cavity; 8, a continuation of the same processes; 9, the endoderm 

 is now a single layer of cells and the gastrovascular cavity is formed. This stage 

 now attaches at one end. The mouth opening and nematocysts form, thus com- 

 pleting the hydroid development. 



All figures drawn to the same scale. 



vegetal pole (Fig. 6). This difference between the length of the 

 cells persists throughout the later stages of development. It is 



