518 CHARLES W. HARGITT 
as those of Pennaria, Hydractinia and Turritopsis, where the 
freedom of the egg from all influence of gonophore walls, etc., 
ought to afford perfect conditions as to regularity of cleavage. 
It may not be improbable that external conditions of pressure, 
etc., have an appreciable influence on cleavage, but such facts 
as those just cited clearly indicate that there are other factors 
concerned which are probably more potent than the merely phys- 
ical ones of pressure, gravity, ete. 
6. The morula. As in Hydractinia and Pennaria, the early 
embryo in Clava is a morula. Cleavage results in a solid mass 
of cells, with only incidentally a sign here and there of an inter- 
cellular space, and in only rare instances anything comparable 
with a segmentation cavity. Indeed, one might venture to aver 
that such cavity is conspicuously absent throughout the ontogeny 
of this hydroid. As already pointed out, this is not peculiar, but 
rather the general fact in hydroid development. Something 
further will be said on this point in a later connection. There is 
nothing in the morula stage in Clava which differs appreciably 
from that of the other species already referred to. As the embryo 
reaches the morula condition it assumes the usual spherical shape, 
whatever may have been the shape of the egg during cleavage 
or growth. Evidently the walls of the gonophore do not afford 
any very serious barrier to this change, for one finds all condi- 
tions of shape from the flattened lateral pocket of the growing 
oocyte to the spherical terminal capsule, and the oval capsule of 
the planula, all derivable in turn from the first as the embryo 
grows and finally emerges as a pear-shaped planula. 
7. The germ layers. What has been said on this subject in 
connection with Hydractinia may be affirmed of Clava. Granted 
the assumption of a morula as the primitive embryo, andthere 
is no occasion for question or discussion concerning the segmen- 
tation cavity, delamination, multipolar immigration, etc. Abso- 
lutely nothing of these is involved in the case under consideration. 
At the time of the completion of cleavage,—indeed before this, 
when the morular aspect first begins to take shape,—we have 
only a spherical cell mass, with syncytial tendencies, and as yet 
without sign of tissue differentiation. In fig. 29 is shown such 
