XXI ^ DEVELOPMENT 233 



As the ovum grows yolk-spheres (Fig. 52), small rounded 

 masses of proteid material, are formed in it, and in Hydra 

 viridis it also acquires green chromatophores. 



When the ovary is ripe the ovum draws in its pseudopods 

 and takes on a spherical form : the investing layer then 

 bursts so as to lay bare the ovum and allow of the free access 

 to it of the sperms. One of the latter conjugates with the 

 ovum, producing an oosperm or unicellular embryo. 



The oosperm divides into a number of cells, the outer- 

 most of which becomes changed into a hard shell or capsule. 

 The embryo, thus protected, falls to the bottom of the water, 

 and after a period of rest develops into a Hydra. As, how- 

 ever, there are certain abnormal features about the develoi> 

 ment of this genus which cannot well be understood by the 

 beginner, it will not be described in detail, but the very 

 important series of changes by which the oosperm of a 

 multicellular animal becomes converted into the adult will 

 be considered in the next lesson. 



