1028 The American Naturalist. [November, 
spermatozoa, May, June, and July. Bisexual. The eggs fall off from 
the mother as soon as the outer coat of the embryo is formed. The 
embryos do not hatch for one or two months. A. fusca. Eggs 
mature in October. Bisexual. The eggs do not fall off, but are stuck 
to surrounding objects by means of a jelly, and flatten down somewhat 
on the support. Probably the eggs are deposited in different places. 
The time between egg-laying and hatching is as in the last species. 
H. sp. ? Adult resembles closely Æ. fusca. More than half of the indi- 
viduals kept in the aquarium developed into males alone, and later 
after the disappearance of the testes they did not develop eggs, but 
continued to bud. The remaining individuals developed into females. 
The eggs remain attached to the mother. When all of the eggs have 
developed as far as the two-layered stage the mother contracts strongly 
her body down to the base. The eggs come to lie around the base of 
` the mother, remaining sticking to her. The mother remains in this 
strongly contracted condition for several weeks, and the embryos often 
escape from the shell before the parent again extrudes. Three embryos 
hatched in one case fourteen days after the contraction had taken 
place. 
The author suggests that the adult species may be identified in the 
form of the eggs and the structure of the egg-case : 
1. Hydra viridis. Egg falls off. Form spherical, Case smooth. 
2. Hydra grisea. Eggs fall off. Form spherical. Case covered with 
large and often branched spines. 
3. Hydra fusca. Eggs singly stuck to objects. Form below, flat ; 
above, convex. Case covered with spines only on the upper surface. 
4. Hydra sp.? Eggs (at base of mother) generally all stuck at one 
place. Form spherical. Case covered with short spines. 
The maturation and fertilization of the egg is described in much- 
greater detail than ever before. The egg begins to round up, drawing 
in the protoplasmic processes, but still remaining beneath the ecto- 
derm. At this stage the zwo polar bodies are extruded. The egg 
later breaks through the overlying ectoderm, but remains attached by 
a broad base. Fertilization then takes place. 
The eccentric position of the segmentation sucleus - causes the first 
furrow to begin at the distal pole (away from the mother), and then, 
after many changes in the outline of the egg, the yolk follows the 
nuclear division, and moreover, in the same way as Bergh? describes in 
Gonothyraa,—viz., the nucleus divides a second time, and the second 
M Jahrbuch, Val. V., 1879. 





