350 BR. N. ANNANDALE ON 



they are confined to the upper part of the body causes the species to resemble Hydra vir- 

 idis, H. fusca and H. grisea, rather than H. dioecia, in at least one respect ; but in the her- 

 maphrodite species the upper testes are the most immature. The development of a 

 bud at the lower limit of a vertical row of spermaries supports Lang's l views, based on 

 histological research, as to the homology between buds and testes in the genus. The 

 development of the spermaries appears to be slower than it is in some forms of Hydra, 

 and I have not been able to see, working with living specimens, the walls of the elon- 

 gated epidermal cells which divide up the lumen of the spermary in some forms ; but it 

 is probable that they exist until the active spermatozoa break them down. 



I have not observed the fertilization of the ovum, but there is no reason to regard 

 it as in any way abnormal. 



Ovary — 



The ovary is a minute globular body seated deep in the ectoderm and resembling 

 that of Hydra dioecia. It persists after the production of an egg and possibly produces 

 a second egg under natural conditions, although I have not known it to do so in captive 

 specimens. On one occasion I saw an ovary which had not yet produced an egg, dis- 

 integrate owing to the wandering apart of its component cells, which appeared to join 

 those of another ovary higher up on the trunk of the parent. The latter organ had al- 

 ready borne an ovum. The eggs (if more than one is produced) are borne on two op- 

 posite quadrants of a transverse section of the body ; but not in any definite order, ex- 

 cept that the uppermost ones usually mature first. They are confined to the upper half 

 of the body. Two may be produced in one quadrant before the appearance of a third 

 in the other. 



The egg first becomes visible to the naked eye as an intensely white, star-shaped 

 body lying at the base of the ectoderm. It increases in size rapidly, gradually draws in 

 its pseudopodia (the rays of the star) and makes its way through the ectoderm to the 

 exterior. The process occupies not more than two hours. The issuing ovum does not 

 destroy the ectoderm cells as it passes out, but squeezes them together round the 

 aperture it makes. Owing to the pressure it exerts upon them, they become much 

 elongated and form a cup, in which the embryo rests on the surface of the parent. By 

 the time that the egg has become globular, organic connexion has ceased to exist. The 

 embryo is held in position partly by means of the cup of elongated ectoderm cells and 

 partly by a delicate film of mucus secreted by the parent. 



Before segmentation conmences, a thin but extremely opaque membrane is secre- 

 ted over the egg, apparently by the ovum itself, and the process of development is thus 

 hidden. The surface of this membrane has a tesselated appearance. 



In from twenty-four to forty-eight hours after the egg has issued through the ecto- 

 derm, its outer membrane commences to break up, and it is not until it does so that it is 

 possible to distinguish a thick-shelled from a thin-shelled egg. 



If the egg belong to the thick-shelled variety, it can be seen by the naked eye when 



1 Zeitschr.f. wiss. Znol., LIV, 1892, p. 37 



