THE SUB-KINaDOM C(ELENTERATA. 



16 



composing the bulk of the ovum, may be perceived 

 a small cavity, the ' germinal vesicle,' which, in 

 its turn, contains a still smaller particle, the ' ger- 

 minal dot ' {fig. 2, h). In addition to these parts, 

 many ova are provided with an outer envelope, 

 known as the yolk-sac or ' vitelline membrane.' 

 The spermatozoa vary much in form. More com- 



Development of Ccelenterata : — a, spermatozoa of Ccelente- 

 rata-, by section of Ccelenterate ovum, with germ-yesicle and germ- 

 spot ; c, ovum after segmentation ; d, section of the same, more 

 , advanced, showing its division into two layers ; e, longitudinal 

 section of typical Hydrozoon ; / and g, longitudinal sections of 

 typical Actinozoon, in different stages of development. (These 

 drawings are diagrammatic.) 



monly they appear as delicate filaments, swollen 

 at one extremity into a somewhat oval body (a). 



After fecundation, the ovum exhibits a series of 

 changes inaugurated by the process of * segmenta- 

 tion ' or yolk-division. 



First, either the whole or a portion of the yolk 



