DEVELOPMENT OF HTDBA. 55 



animals. They arise, as in other higher animals, from a 

 self-division of the nuclei of the testis-cells. There is a ti-ue 

 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 monoecious or hermaphro- 

 dite — i.e., the sexes are not distinct. The egg of Hydra 

 originates from the central cell of the ovary. 



Thei'e is a true segmentation of the egg. The young 

 Hydra thus passes through a trvie 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 iirst 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 

 tlie two, afterward three, definitive germ-lamellaj, common 

 to all animals except the one-celled Protozoa. 



Tlie 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, 



