HYDRA, AND HYDROID ZOOPHYTES. 131 
for into whatever number of parts its body may be cut up, 
each, under favourable circumstances, can give origin to 
a new and entire polype, so that thirty or forty individuals 
may thus be produced by the division of one. 
123. The proper generative process, here reduced to its 
utmost simplicity, consists in the development of a germ-cell 
and of sperm-cells in the substance of the wall of the stomach, 
the former being produced near the footstalk, the latter just 
beneath the arms. The egg which is evolved irom the former, 
being fertilized by the products set free from the latter, gives 
origin to a young Hydra, which resembles its parent. The 
two reproductive processes, however, are performed under 
very different conditions ; 
for whilst multiplication by 
gemmation is favoured by 
warmth and a copious sup- 
ply of food, the true gene- 
tative process seems to be 
brought about by a lower- 
ing of the temperature, and 
to have for its object the 
tuation of the race 
through the winter, the egg 
being capable of enduring a Se 
degree of cold which would 
be fatal to the polype itself. 
124. The group of Hy- 
drozoa is for the most part 
made up of composite fabrics 
more or less resembling the 
Campanularia (fig. 72), 
which may be likened to 
a Hydra whose buds do not 
_ detach themselves, but re- 
- main in connexion with the 
_ stock that produced them ; iy See eee 
_ the whole plant-like struc- Fig. 72.—CamPANULaRiA. 
_ ture, moreover, being strengthened by the consolidation of its 
_ external layer into a horny sheath, which retains its form 
_ after the destruction of the soft parts. Thus each comes to 
consist of a stem and branches, on the sides or ends of which 
? K2 
