HYDRA 
5 
travels to the other cell and unites with the protoplasm there. 
The essential thing in sexual reproduction is the union of the 
nuclei from two cells. In the case of the spirogyra they are 
common growing cells. The combined nuclei 
form one cell which after resting a while de¬ 
velops and so becomes a new individual. 
1. What is the essential process in sexual repro¬ 
duction ? 
2. How does the protoplasm get from one spi¬ 
rogyra cell to the other ? 
Hydra. — This small animal, about the di¬ 
ameter of a pin and one fourth of an inch 
long, found clinging to water weeds, well il¬ 
lustrates sexual reproduction among the lower 
animals. The reproducing cells are not com¬ 
mon growing cells as in spirogyra, but special 
cells set apart for reproduction. The larger 
of these two cells is called the egg cell or ovum 
(plural, ova ); the smaller is called the sperm 
cell or spermatozo'on (plural, spermatozo'a ). 
The spermatozoa are produced in enormous 
numbers in a part of the hydra called the 
spermary. When they become mature, they Figure 6.—Two 
break out and swim about in the water. spirogyra S Re F 
Some of them by chance come near the ovum producing. 
and are attracted to it. One of them succeeds The protoplasm 
in penetrating the ovum and uniting with its of the growing 
nucleus. This process is called fertilizing the cel1 is arranged 
I i 
1 
1 
igiL 
M 
ft 
2f 
ovum, for without it the egg would not grow. 
in a spiral band. 
1. How do the reproducing cells of the hydra differ from those of 
the spirogyra ? 
2. What is an ovum ? 
3. What is a spermatozoon? 
4. What is fertilization? 
5. Why need the spermatozoon have the power of movement? 
