ANATOMY AND DEVELOPMENT OF HYDRA. 13 



beneath it, then of a third, and so on, so that at last the 

 germ, instead of being surrounded by a layer of naked pris- 

 matic cells, is surrounded by a thick laminated hard shell of 

 chitine. 



After the outer shell is thus formed there is produced 

 between this and the closely-applied germ a second covering 

 consisting of a colourless, transparent, and very elastic pel- 

 licle. Of the origin of this Kleinenberg can say nothing 

 positive, though he thinks it probable that it is caused by 

 the hardening of a liquid secreted between the germ and the 

 first-formed shell. 



Kleinenberg has convinced himself that the outer germ- 

 shell of Hydra is not due to the secretion and subsequent 

 hardening of a liquid, but that it consists in a total meta- 

 morphosis of the entire outer cell-stratum of the germ. Every 

 one of its component elements is thus a cell, which, as the 

 result of the conversion of its protoplasm into chitine, loses 

 its vitality, its proper physiological value, yet retains its 

 morphological equivalence. The shell is therefore an epi- 

 dermal formation, and in relation to the entire germ is one 

 of its tissues. 



The outer germ shell in Hydra vulgaris differs in some 

 important points from that of H. viridis, for instead of the 

 thick shell, with its smooth surface, which occurs in the latter, 

 it consists in H. vulgaris of a thin chitinous capsule, which 

 is set round with a multitude of irregular spines mostly cleft 

 at their free extremities. This peculiar form is due to the 

 origination in each of the superficial prismatic cells of a large 

 vacuole immediately below the free surface of the cell, the 

 rupture of the cells over the vacuole, the fusion of several 

 vacuoles into one, the consequent formation of a sort of 

 labyrinthine tissue, and the conversion of this into the 

 chitinous shell. 



It appears, then, established that the first differentiation 

 of the germ of Hydra consists in the formation of a peripheral 

 cellular lamina, the protoplasm of whose cells becomes con- 

 verted into chitine, and thus forms a solid shell. The first 

 organ which is developed from the germ is thus a provisional 

 embryonic one, which takes no part in the formation of the 

 definitive body, and on the liberation of the young is simply 

 cast off". 



The period of the proper embryonal development takes a 

 far greater time for its accomplishment than has been needed 

 for the processes just described. While the whole of the 

 processes from the first appearance of the egg to the com- 

 pletion of the germ-shell, are mostly completed on the fourth 



