118 OUTLINES OF EVOLUTIONARY BIOLOGY 



which will serve later on for the nutrition of the developing 



embryo. 



Some of the surrounding cells of the ectoderm at first 



form a covering 

 or envelope for the 



/- 



M 



-hyth. 



growing ovum, 

 but this is pre- 

 sently ruptured 

 and the mature 

 egg is exposed on 

 the surface of the 

 B body of the parent 



Hydra. There it 



is found by a spermatozoon, which is 

 attracted towards it and by its own 

 activity bores its way into the ovum. 

 This act of fertilization is concluded in 

 the usual manner by the fusion of the 

 nucleus of the spermatozoon (male pro- 

 nucleus) with that of the ovum (female 

 pronucleus) to form the zygote nucleus. 

 The fertilized ovum or zygote (Fig. 

 59, A) undergoes segmentation while 

 still remaining attached to the parent 

 Hydra. In this way a single-layered, 

 hollow blastula (B) is formed, which 

 becomes converted into a two-layered 

 embryo by migration of cells into the in- 

 terior to form the at first solid hypoblast 

 or endoderm (C, D, ?'). The epiblast 

 or ectoderm cells (D, ec) now secrete a 

 thick horny protective envelope (sh, 

 shi) around the embryo, w r hich falls 

 off from the parent and undergoes a 

 period of rest at the bottom of the 

 pond. After a time the interrupted 

 development is resumed, the horny envelope is ruptured, and 

 the embryo escapes (F). The gastral cavity appears in the 

 midst of the endoderm cells, the mouth is formed by perforation 

 at one end, and the tentacles bud out. "With the formation of 

 the mouth the gastrula stage is reached, but it will be noted that 



A. 



FIG. 60. Obella yemculata. 



A, part of hydroid colony ; B, 

 free - swimming medusa ; 

 both x 13. (From photo- 

 graphs.) 



bst., blastostyle; gon., gonad; 

 gth., gonotheca ; liyc., hy- 

 drocaulus ; hyd., hydranth ; 

 hyth., hydrotheca ; mn., 

 manubrium ; p.s., perisarc ; 

 r.c., radial canal ; ten., ten- 

 tacles. 



