92 



ORGANISMS OF TISSUES 



mouth which thus becomes the most sensitive or irritable part 

 of the body. 



D 



FIG. 38. The egg of Hydra and its development. A , The mature ovum full of 

 yolk granules; B y section of blastula formed by segmentation of the ovum; C, 

 formation of the inner mass of cells by transverse division of the outer cells; Z>, 

 solid mass of endoderm and ectoderm cells, and cyst-like outer membrane ;^ F, 

 embryo emerging from membrane; E, discarded membrane and G, separation 

 of endodermal cells to form the enteric cavity. (From Dendy, after Bourn: 

 and Brauer.) 



D. REPRODUCTION. The naturalist Trembley living in the 

 i8th century discovered that a Hydra might be cut into many 



