34° 



ANIMAL DEVELOPMENT 



animals, as contrasted with still higher forms which, as we shall 

 see, are essentially three-layej'ed. 



During the summer months Hydra propagates by means of 

 budding, but as the autumn approaches, one or more rounded 

 thickenings of the ectoderm appear near the attached end, and 

 several pointed thickenings of the same layer near the mouth end 

 (fig. 860). Each of the former is an ovary, and each of the latter 

 a spermary. Within the ovary there are at first a number of little 

 cells, one of which becomes bigger than its fellows, and uses these 

 as food after the fashion of an Amoeba. It has now become 



YOLK 



GRANULES 



ENDODERM 

 Fig. 861. — Development of Freshwater Pulype [Hyffra], enlarged 



t, Ovum in amceboid stage. 2, Ripe ovum projecting from body-wall of adult 3, Section through egg- 

 shell. 4. Section through embryo in two-hiyered stage {shell omitted). 5, Section through ripe embryo 

 escaping from shell (cells of ectoderm and cndoderm not shown.i. 



an egg-cell or ovum, within which are to be seen a number of 

 granules of nutritious substance (yolk) which serve as food to the 

 developing embryo. After a time the covering of the ovary is 

 ruptured, and the egg-cell projects externally. Each spermary 

 also contains a little mass of cells that divide repeatedly to form a 

 large number of minute tadpole-shaped bodies, the sperms, which 

 are ultimately set free into the surrounding water, where they 

 swim about. Precisely as in Volvox (p. 335) the egg-cell is 

 fertilized by fusion with it of a single sperm, derived from the 

 same or another Hydra. The fertilized egg-cell next surrounds 

 itself with a firm protective coating, and undergoes cleavage to 

 form a mass of small cells {mortiia), which may be called an 

 embryo (fig. 861). This now becomes detached from the body of 

 the parent, and falls into the mud, where it remains in a dormant 

 state during the winter. Since all or most of the adult animals 



