94 



ORGANISMS OF TISSUES 



coelenterates, in the accumulation of nerve cells around the 

 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, section of blastula formed by segmentation of the ovum; C, 

 formation of the inner mass of cells by transverse division and immigration of 

 the outer cells; D, solid mass of endoderm and ectoderm cells, and cyst-like 

 outer membrane; F, embryo emerging from membrane; E, discarded mem- 

 brane; and G, separation of endodermal cells to form the enteric cavity. 

 (From Dendy, after Bourne and Brauer.) 



D. REPRODUCTION. The naturalist Trembley, living in the 

 i8th century, discovered that a Hydra might be cut into many 

 pieces, and that each piece would continue to live and would 



