HYDRA: ITS ANATOMY AND DEVELOPMENT. 271 
ing. The diameter of the egg of H. vulgaris is one-fiftieth of an 
inch ; that of A viridis one-sixty-sixth of an inch. When the 
ovum is expelled from the ovary it is very transparent and soft, 
and remains attached to the body of the Hydra for some days. 
It is then that the spermatozoa escapes from the testis and fer- 
tilize the ovum. The egg is at first a cream colour, but changes 
to orange colour. When detached from the Hydra the egg sinks 
to the bottom of the water, and soon becomes covered with ex- 
traneous matter, through which it cannot be seen, and by which it 
is protected from pressure until it hardens. 
As a rule, after the extrusion of the ovum, the Hydra slowly 
dissolves and disappears. But I have had exceptions, when the 
Hydra has only partly dissolved after producing the ovum. In 
one case in which & vu/earis had been divided in February of 
this year, it healed up again, and in March produced ovum and 
spermatozoa. After expelling and fertilizing the ovum, it turned 
itself inside out until the tentacles could not be seen, and then 
expelled a second egg and fertilized it. After this performance the 
head dissolved, but the lower part of the body lived and budded, 
after which performance it formed a new head. This is, undoubt- 
edly, a rare Occurrence. 
As arule & viridis produces the ovum in the spring, H. fusca 
in the summer, and A. vulgaris in the autumn, but the above 
order is not always maintained, for I have seen the ovum of Z. 
vulgaris in the latter part of the winter, and you will see this even- 
ing HZ. viridis producing the sperm ducts, and, in all probability, the 
ovary and the ovum. 
The eggs are supposed to be hatched in the following spring ; 
but I find that in confinement this is not the case, and that they 
are all hatched in a few weeks. As the time for hatching ap- 
proaches, the envelope becomes thinner at one part, in consequence 
of the egg being pushed out at that side (fig. 64). This I observed 
on April 26th, and it was forty-nine days after expulsion. One 
hour after this appearance, a small opening was seen in the shell, 
the egg appeared to alternately contract and expand until, at last, 
the tentacles of the young Hydra appeared, as shown in fig. 6s, 
three hours after the crack was first seen. For a few hours this 
young Hydra continued to develope, although but slowly. In 16 
hours it was perfectly formed, although very minute. It had six 
tentacles, and when fully extended appeared as in fig. 66. It re- 
mained attached to the shell for three days, continually contracting 
and expanding. In appearance it was very transparent and snowy 
white. After leaving the egg, the growth of the young Hydra is 
slow. It appears to take no food for some weeks. 
The number of the tentacles vary from three to seven. I have 
in some cases observed the young Hydra bud before they partook 
