No. 395-] PENNARIA TIARELLA McCRADY. 865 
slight papilla-like elevation on the side of the hydranth. The 
endoderm goes through a similar process, though not so exten- 
sive. There is no open cavity in the bud at this stage (Fig. 1). 
The space is entirely filled by the cells of the endoderm. The 
cells of the ectoderm have changed in appearance very much 
from those adjacent to them in the hydranth proper. These 
cells are the chief seat of activity, as all of the more important 
changes, which occur from now on, take place 
in them. The cells of the endoderm are 
secondary in importance. These results are 
confirmatory of the work of Dr. F. Braun (3) 
and contrary to that of Arthur Lang (4). The 
cellular structure in the ectoderm. has practi- 
cally disappeared at this early stage, and we have a large 
number of nuclei scattered in the cytoplasm. These nuclei 
are large and prominent. They are nearly spherical in shape. 
There is a very definite nucleolus, usually centrally located, 
which stains very deeply. The nucleoplasm has a few chroma- 
tin fibres irregularly arranged. This condition of the ectoderm 
is brought about in the following manner. The ectoderm, more 
especially at the distal end, begins to grow by a proliferation 
of cells, so that the thickening of the ectoderm, which takes 
place in the first stages in the development of the Medusa, is 
not so much a thickening as it is an increase in the number of 
œ cells in this region. The bud begins now to 
sm, elongate until it has become partly differen- 
-E tiated from the hydranth (Fig. 2). The endo- 
dermal cells are smaller than at the earlier stage 
and enclose a cavity. The proliferation of 
the cells of the ectoderm continues until the 
bud has become about half full. It would be 
more accurate to say that the nuclei become more numerous 
than that the cells increase, because there is no indication of 
cells other than the presence of the nuclei and the cytoplasm 
in which they are imbedded. This may, however, be due to 
the fact that an absence of cell membrane makes difficult and 
indistinct the cell boundaries. While this change has been 
taking place, the Medusa has grown larger and longer. The 
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