HYDRA 



117 



regarded as potential ova, being sacrificed for the benefit of the 

 one ; in fact they are simply devoured by the voracious egg 



FIG. 59. Development of Hydra. (From Bourne's " Comparative Anatomy," 

 partly after Brauer.) 



A, the mature ovum, full of yolk granules and still attached to the body wall of the 

 parent ; B, section of blastula or blastosphere produced by segmentation of the ovum ; 

 C, the embryo becoming solid by migration into the blastocoel of cells cut off from 

 the wall of the blastula to form the hypoblast ; D, solid embryo composed of epiblast 

 and hypoblast and enclosed in a protective shell ; E, embryo flattening itself out 

 within the shell ; F, embryo emerging from the shell and with the gastral cavity 

 appearing in the endoderm (hypoblast) ; G, empty shell after the escape of the 

 embryo. 



blc, blastocoel ; ec, ectoderm (epiblast) ; en, endoderm (hypoblast) ; i, solid mass of hypo- 

 blast cells ; mg, mesogloea ; sh, shell ; she, outer layer of shell ; sJii, inner layer of shell. 



cell, which puts forth pseudopodia and feeds upon them like a 

 hungry Arnreba. In this way the ovum attains a relatively large 

 size and its cytoplasm becomes loaded with yolk corpuscles 



