158 



HARDWICKE'S S C IENCE - G SSIP. 



On Oct. 30 the egg (6g.l06, ill) became detached 

 from the Hydra, and sank to the bottom of the cell ; 

 it then presented the appearance of fig. 100. It was 

 at first of a cream-colour, which soon changed to 

 orange. It should be noticed that the gelatinous 

 envelope frequently becomes covered with extra- 

 neous matter, through which the egg cannot easily 

 be seen; this may account for the difficulty of 

 obtaining the ova, from ponds and ditches, as they 

 are so well concealed from observation. The col- 



Fiff. 106. Hydra; K, sperm-cell*, of which there are three ; 

 L, discharge of spermatozoa ; M, ovum about to leave the 

 body. 



lection of substances around the eggs, acts, I 

 believe, as a protection from pressure, for on leaving 

 the Hydra, they are very soft, and easily crushed. 

 I have seen small Cypris burst them at an early 

 period ; but they soon become hardened so as to 

 resist pressure, and can then be removed with a 

 dipping-tube for observation. 



The ovum is at first globular, but if it falls in its 

 soft state on a flat hard substance, the underside 

 becomes flattened, making it hemispherical or 

 helmet-shaped: this does not hinder its proper 

 development. 



As the time for hatching approaches, the envelope 

 surrounding the egg becomes irregular (fig. 101, D), 

 and the egg is slightly pushed out on one side 

 (fig. 101, E). In the specimen above mentioned this 

 change was observed on January 24th, fifty-five days 

 after extrusion; in a few minutes a slight crack was 

 seen in the shell, and a portion of the young Hydra 

 slowly emerged from it, in a rounded form, as shown 

 in fig. 102, F, sketched two hours after the first per- 

 ceptible crack in the egg. It continued slowly 

 emerging, and in two hours afterwards rudiments 

 of tentacles appeared, as rounded lumps (fig. 103, G). 

 Seven hours after the first rupture of the egg, the 

 tentacles had progressed to the condition shown in 

 fig. 104, II, and in twelve hours the Hydra was fully 

 developed, with seven tentacles, and in all particulars 

 like the adult, size only excepted. Its appearance 

 was most interesting, — delicately pure, and beauti- 

 fully transparent, as if made of crystal, and still 

 attached to the inside of the shell by the suctorial 

 disc at the posterior end of the body (fig. 105, J). 

 Some specimens finally leave the shell about twelve 

 hours after being fully developed, others twenty-four, 

 or even sixty hours afterwards, when they fix them- 

 selves to the bottom or sides of the cell. Their 

 growth is very slow, and I could not discover what 

 they took as food. After a month had elapsed, I 

 introduced some small entomostraca, but though 

 they seized them with their tentacles, they could 

 not absorb them. The entomostraca died, however, 

 from the effect of the stinging power of the tentacles. 

 The young have not all the same number of ten- 

 tacles ; one observed had five, most had six, and 

 some few had seven. 



After the extrusion of the ovum, the parent Hydra 

 gradually diminished, the tentacles shortened and 

 slowly disappeared, and in about twenty-one days 

 the whole body dissolved. The sperm-cells, three in 

 number, continued on the body for some days after 

 the ovum had been separated from it, and con- 

 tinued to discharge spermatozoa into the water. 



Both ovi-sac and sperm-cells are usually found on 

 the same Hydra; but sometimes sperm-cells only 

 are found, when the whole length of the body is 

 studded with them. I have counted in some cases 

 eleven, in others seventeen, and in one as many as 

 twenty-three ; but where an ovum is formed the 

 sperm-cells rarely exceed four in number. 



The reproduction of Hydra vulgaris from ova 

 takes place in the autumn, and that of Hydra viridis 

 in the spring. 



"The most important organic substances met 

 with in Nature are those in which carbon is asso- 

 ciated in various multiple proportions with the 

 elements hydrogen and oxygen ; next in importance 

 are those containing nitrogen in addition to these ; 

 sulphur and phosphorus are also present in some 

 few.'"' — Armstrong's Organic Chemistry. 



