866 THE AMERICAN NATURALIST. [VoL XXXIII. 
filling up of the distal end of the bud with these prominent 
nuclei and the surrounding cytoplasm at first forces the endo- 
derm back, but it soon begins to force its way 
down through these cells in the center and 
around the margin of the bell. The prolon- 
gation of the cells which grow down in the 
center is to become the manubrium. Jointly 
with this formation, the endoderm is sending 
a layer of cells around the bell close to the 
ectoderm (Fig. 3). As these cells grow 
around, they cut off the interior mass of 
ectodermal cells from the outside layer, as shown in Fig. 4. 
This layer of endoderm now becomes 
the second layer in the umbrella of the 
Medusa. All of the cells between the 
manubrium and this new layer are of 
ectodermal origin. The cells adjacent 
to this layer of endoderm show a ten- 
dency to arrange themselves in a row, 
the nuclei of which are smaller than 
those from which they have been derived. 
By the time the endoderm has grown 
around, gua separating the outer layer of ectoderm 
from the cells within, this 
row of cells has become a 
definite layer. This layer 
becomes now the lining of 
the bell. When the layer 
is completely formed, the 
nuclei are much smaller 
than they were when the 
layer was forming. This 
larger layer is more prom- 
inent in P. cavolini; the 
cells are larger and the nu- 
clei more numerous. The 
mass of cells formed be- 
tween the bell and the manubrium is to give rise to the 
Fic. 4. 
