264 J. FULLAGER ON DEVELOPMENT OF Hydra Vulgaris. 



hatcliing. The egg of H. viridis has at first also an envelope, 

 which disappears some time before hatching. In fig. 1, c, I have 

 particularly marked the triangular-shaped darker spots in the 

 envelope ; they appeared as regular as in the sketch, a little denser 

 than the rest of the envelope, and surrounding the whole of the 

 egg. This, however, can only be seen at the margin, but on 

 moving the egg in the water, it may be seen on every part, and 

 always presenting the same appearance. The diameter of the egg 

 of H. vulgaris is -^^ of an inch, that of H. viridis -^^ of an inch. 



On October 27th, 1873, I placed in a glass cell a specimen of 

 H. vulgaris^ on which an ovum had begun to form. It was put 

 into pure clean water by itself, so that nothing obstructed the view, 

 and the changes which took place from time to time were very 

 clearly observed. Besides the ovum, it had three fully-developed 

 sperm cells (pi. xiii., fig. 7, K.), in which spermatozoa were seen 

 in active motion, and from each of which, at intervals, a quantity 

 was discharged into the surrounding water (fig. 7, Z).* 



On October 30, the egg (fig. 7 M) became detached from the 

 Hydra, and sunk to the bottom of the cell ; it then presented the 

 appearance of fig. 1. It was at first of a cream colour, which soon 

 changed to orange. It should be noticed that the gelatinous enve- 

 lope frequently becomes covered with extraneous matter, through 

 which the egg cannot easily be seen ; this may account for the 

 difiiculty of obtaining the ova from ponds and ditches, as they are 

 so well concealed from observation. The collection 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 re- 

 moved 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 

 develoj)ment. 



As the time for hatching approaches, the envelope surrounding 

 the egg becomes irregular (fig. 2 D), and the egg is slightly pushed 

 out on one side (fig. 2 E). In the specimen above mentioned this 



* The spermatozoa were originally made out by Mr. Gulliver, F.R.S., who 

 obtained specimens of the Hydra from me, and executed drawings of the 

 spermatozoa, which are engraved in " Science Gossip," vol. for 1873, pages 13 and 

 41, and are the first representation published of these objects in this couutry, if 

 not in the world. My own objectives were not of sufficient power for such delicate 

 investigations, but Mr. Gulliver, with Powtll undLealands' ^, was well prepared. 



