TWO SPECIES OF HYDRACTINIA 
483 
has become distinctly elevated; the opening by which the inner 
ectoderm communicates with the outer is also eccentric in posi- 
tion but has become smaller, and shows signs of being closed up, 
as may be inferred from the presence of a deeply staining substance 
similar to that of the supporting lamella which spans the space 
lying between the edges of the endodermal lamella. The cells 
of the inner ectoderm are not very numerous, and their nuclei 
can be clearly distinguished between the egg cells. The young 
ova that are sometimes found in the endoderm of the blastostyle 
close to the base of the gonophore bud at this stage are probably 
reserved for the next bud. In the final stage here reproduced 
(fig. 14) in which some of the ova are nearly ripe, the gonophore 
is nearly spherical, the spadix is very conspicuous and may some- 
times show a tendency to branch and grow outward between the 
egg cells. The endodermal lamella has become exceedingly thin 
and the nuclei contained in it are recognizable with extreme diffi- 
culty, so that at such an advanced stage the presence of an endo- 
dermal lamella can hardly be made out. The failure on the part 
of previous observers to recognize the existence of the endodermal 
lamella in Hydractinia echinata is probably due to thefactthat 
it becomes exceedingly thin at a very early stage. The opening 
at or near the top of the gonophore b}- which the inner and the 
outer ectoderm communicate with each other is probably never 
closed entirely ; for in a nearly ripe female gonophore examined 
to settle this point, there was found an opening at the place in 
question which ran through two sections (each = 7.5 fi). The 
opening occupied the centre of a septum consisting entirely of the 
supporting lamella, the first formation of which was seen in a 
previous stage (fig. 13). Another fact that must be noted in this 
stage is the great increase in number of the cells of the inner ecto- 
derm and their different appearance. So far these cells had a 
naked protoplasm, but now most of them have developed a dis- 
tinct cell wall, and by mutual pressure have assumed a poly- 
hedral form. The few cells which are still seen in fig. 14 to be 
destitute of a membrane probably develop it later. Frequently 
the egg cells are separated from one another by wide spaces com- 
pletely filled with these cells, which thus almost deserve the name 
