346 
SAMUEL RITTENHOUSE 
the different blast omeres during the early cleavages, are frequently 
encountered. The segmenting cells arrange themselves around 
a continually enlarging cleavage cavity. 
5. At the completion of segmentation a true blastula is formed, 
which develops cilia and swims with a spiral motion. The oval 
blastula elongates and is transformed into a planula. 
6. The ectoderm arises directly from the segmentation cells 
which are arranged in a peripheral layer around the blastocoele. 
7. The formation of the entoderm is by unipolar ingression. 
The cells at the posterior end of the blastula bud off the primitive 
entodermal tissue w^hich migrates into the blastocoele; and later 
is arranged into the inner germ layer. 
8 . Nematocysts arise chiefly in the interstitial cells, sometimes 
in the entoderm, and migrate to the surface. 
9. The larva becomes attached by its side and is transformed 
into the hydrorhiza. The root frequently branches soon after 
attachment. 
10. The hydranth develops from a bud, which is given off 
from about the center of the hydrorhiza. 
11. The tentacles appear early as small projections at the 
distal part of the hydranth bud. 
12. A thin, delicate perisarc is secreted around the hydrorhiza 
and stem up to near the tentacles. 
