268 



COMPARATIVE ANATOMY 



Fig. 56. 



Development of Hydra. A, the ovum filled with deutoplasts ; ec, 

 ectoderm of the parent Hydra; nig, mesoglcea ; en, endoderm ; 

 B, blastula stage, in which some cells of the blastula wall are 

 passing in towards the interior ; blc, segmentation cavity or 

 blastoccele. C, the blastocoele nearly filled up by immigrant cells. 

 D, an embryo further developed with sh, an external spiny, and shi, 

 an internal protective envelope ; ec, ectoderm; /, inner solid mass of 

 cells. E, the embryo still enclosed in its double protective case, 

 flattening itself out. P\ the embryo creeping out of its protective 

 envelopes ; lettering as in A and 1). G, the empty envelope after 

 escape of the embryo. (D, original ; the rest after Brauer.) 



