EMBRYOLOGY OF STOMOTOCA APICATA 
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cells are more or less definitely placed around the segmentation 
cavity. The blastomeres finally become very numerous and 
small, and arrange themselves around the blastocoele in a single- 
celled layer forming a true blastula. 
BLASTULA 
The blastula is oval in shape, and is but slightly larger than the 
unsegmented egg. The average size of several blastulae that were 
measured was .19 mm. in length and .15 mm. in their largest trans- 
verse diameter. The egg before cleavage measured, as stated 
before, .14 mm. in diameter. The blastomeres in the blastula 
stage have become very numerous and small, and arranged in a 
single layer of epithehal cells. When the larva is about eight or 
ten hours old, these peripheral cells develop cilia; probably each 
cell has one cilium. With the development of the cilia, movement 
commences. At first the motion is very slight, but as the cilia 
become more numerous, the blastula is enabled by the ciliary 
movements to leave the bottom of the aquarium upon which it 
lay and to swim about in the water with a spiral or cork-screw 
motion that is characteristic of hydroid blastulae and planulae. 
The large end of the blastula is directed forward and therefore 
may be called the anterior end. Whether the anterior part of 
the larva corresponds to the upper or to the lower pole of the egg 
was impossible to determine. It is reasonable, however, to infer 
that there may be no fixed polarity in the Jarva of Hydromedusae, 
for it is well known that normal embryos of small size will develop 
from fragments of eggs. 
PLANULA 
The blastula gradually elongates and becomes narrower forming 
a larva which is usually about three times as long as broad and 
known as a planula. From measurements taken of living planu- 
lae the average size is about .25 mm. in length and .08 mm. in the 
short diameter. These measurements are not constant, the larva 
becoming somewhat longer at an older age. The anterior end 
