From the cross-section of the _Hydra_ make out the general structure of the body. Note that it is a hollow cylinder consisting of two well-defined layers of cells, an outside _ectoderm_ layer and an inner _endoderm_ layer. Between these two is yet another thin non-cellular layer called the _mesoglœa_. Thus it will be seen that _Hydra_ is made up of two layers of cells, the outer ectoderm or skin, which is specialized to perform the office of capturing prey as well as that of protection, and the inner endoderm, which surrounds the digestive cavity and performs the function of digestion. The endoderm lines the body-cavity, particles taken in as food being digested by certain digestive cells which thrust out amœboid processes and ingest particles of food. Other cells in the endoderm have long flagellate processes which vibrate back and forth in the digestive cavity, thereby creating currents in the water containing food-particles. Note, in a cross-section, that there are small ovoid or cuboid cells at the bases of the large ectoderm cells. These are the _interstitial cells_. Some of the interstitial cells become modified and pushed up between the ectoderm cells to form cnidoblast cells. Many of the endoderm as well as ectoderm cells have muscle-processes which spread out from the base of the cell and which serve to contract and expand the body. TECHNICAL NOTE.--In the specimens which have been collected perhaps two methods of reproduction will be observed. Place healthy _Hydræ_ in a wide-mouthed jar in the sunlight with plenty of water and food. In a few days active budding will take place. Observe the method of reproduction in _Hydra_. Commonly the parent produces small buds, which at first are only evaginations of the body-wall, but which later develop tentacles and a mouth of their own. Subsequently the bud becomes constricted at the base, separates from the parent, and the young _Hydra_ begins a distinct existence. Another mode of reproduction takes place which, in distinction from the asexual method just mentioned, is called sexual reproduction. This last is the method common to most of the higher organisms. You may note that in some _Hydræ_ there is a swelling or bulging of the ectoderm of the body-wall in the region just below the tentacles. These are the _sperm-glands_. Within these are produced sperm-cells which break away in great clusters to fertilize the ova, or eggs. Note a larger bulging of the body-wall nearer the lower end of the body which, under high power, has a granular appearance. This is the _egg-gland_, in which develops a single _ovum_ or _egg_. The ovum breaks from its covering and is fertilized by sperm-cells from another individual. In forms like _Hydra_, where both sexes are represented in a single individual, the organism is termed _monœcious_ or _hermaphroditic_. In connection with reproduction Chapter XIII should be studied.