HYDRA. 



139 



The segmentation which follows is total and equal, and results in the 

 formation of a blastosphere (Fig. 41, 5). By inwandering, or by division 

 of the cells of the blastosphere, an internal endoderm is formed, and this 

 formation takes place on all sides. In a word, it is multipolar. The 

 segmentation cavity of the blastosphere is thus filled up, and the two 

 layers become differentiated from one another. 



The outer or ectodermic layer forms (a) an external "chitinoid" shell 

 of several layers ; (b) an internal membrane, homogeneous, thin, and 

 elastic ; and (c) the future ectoderm of the adult. In Hydra fusca the 

 egg is separated from the parent before the shell is formed, and is 

 fastened by its gelatinous sheath to aquatic plants ; in H. viridis 

 and H. grisea the egg falls off after the outer shell has been formed, j 

 In all species the separation from the parent appears to be followed by | 

 a period of quiescence lasting from one to two months. 



Within the shell, differentiation at length recommences, but it proceeds 



FIG. 41. Development of Hydra. (After BRAUER.) 



i. sp., spermatozoa. 2. Amoeboid ovum ; g.v., germinal vesicle or 

 nucleus ; y.s., yolk spherules. 



3. Ovum with lobecl envelope (sh) around it. 



4. Ovum protruding ; n., the nucleus ; ect., the ruptured ectoderm ; 

 end., the endoderm. 



5. Section of blastosphere Ect., ectoderm ; End., endoderm 

 being formed. 



6. Section of larva. Ect., ectoderm ; End., endoderm ; g.c., gut- 

 cavity ; s/i., ruptured envelopes. 



slowly. Interstitial cells arise in the ectoderm ; a middle lamella is 

 formed ; a gastric cavity begins to appear in the midst of the endoderm. 

 Thereafter the shell bursts, and development proceeds more rapidly. 

 The embryo elongates, acquires a mouth by rupture at the distal (some- 

 times called vegetative) pole. The inner sheath is also lost, and the 

 young Hydra fixes itself and begins to live as its parent or parents did. 

 The multipolar formation of the endoderm is perhaps a primitive 



