DEVELOPMENT OF HYDRA. 55 
animals. They arise, as in other higher animals, from a 
self-division of the nuclei of the testis-cells. There isa true 
ovary formed in the same interstitial tissue of the ectoderm, 
consisting of a group-of cells, which, Kleinenberg states, 
differ entirely in their mode of formation from the ovaries 
(gonophores) of the marine hydroids, which are genuine 
buds. 
It thus seems that Hydra is moneecious or hermaphro- 
dite—i.¢., the sexes are not distinct, The egg of Hydra 
originates from the central cell of the ovary. 
There is a true segmentation of the egg. The young 
Hydra thus passes through a true morula stage. There 
is an outer layer of prismatic cells, forming the surface of 
the germ, and surrounding the inner mass of polygonal 
cells. At first none of these cells are nucleated, but after- 
ward nuclei appear, and it is an important fact that these 
nuclei do not arise from any pre-existent egg-nucleus. 
The next step is the formation of a true chitinous shell, 
enveloping the germ or embryo. After- this, Kleinenberg 
asserts that the cells of the germ become fused together, 
and that the germ is like an unsegmented egg, being a. 
single continuous mass of protoplasm. 
The remaining history of Hydra is soon told. In this: 
protoplasmic germ-mass there is formed a small excentric 
cavity ; this is the beginning of the body-cavity, which. 
finally forms a closed sac. After several weeks the germ. 
bursts the hard shell and escapes into the surrounding wa-. 
ter, but is still surrounded by a thin inner shell., After this. 
a clear superficial zone appears, and a darker one beneath, 
which is the first indication of the splitting of the germ into 
the two, afterward three, definitive germ-lamellz, common 
to all animals except the one-celled Protozoa. 
The embryo soon stretches itself out, a star-shaped cleft. 
appearing, which forms the mouth. ‘The tentacles next ap- 
pear. The animal now bursts open the thin inner shell, 
and the young Hydra appears much like its parent form. 
There is, then, no metamorphosis in the Hydra ; no cili- 
ated planula, as in many other Hydroids, The adult form 
is thus reached by continuous growth. 
