GERM-CELLS. 45 
In manyanimals it is provided with a sheath or membrane 
of its own; in others it remains naked, and in that case 
frequently displays the remarkable movements of pro- 
toplasm. The germ-cells of different classes of animals 
vary considerably in their microscopic dimensions ; 
nevertheless, in the whole animal kingdom, from the 
sponges and polypes up to the mammals inclusive of 
man, they are essentially similar. Nor do non-essential 
differences appear until the primitive germ-cell is more. 
abundantly provided with yelk and albumen, and has 
surrounded itself with a specially thick and perforated 
shell, as in insects and fishes, or with a peculiarly 
formed sheath, in the shape of a double concave lens, 
as, for instance, in some Turbellaria. As a rule, the 
ova are formed in special organs, 
the ovaries. The other sexual 
element, the sperm, contains, as 
its peculiar active constituents, 
the spermatozoa (fig. 4 5), which 
consist of a pointed, elliptic, or i /. 
occasionally of a hook-shaped, FIG. 4 
head, and a thread-like body. As long as the sperm 
is capable of fecundation, the filamentous appendage 
performs serpentine movements, and the development 
of the spermatozoa from cells, as well as the comparison 
of their movements with the vibrating movements 
of ciliated and flagellate cells, enable us to recognize 
them also as modified cell structures. 
The vehement dispute of last century between Evo- 
lutionists and: Epigenists has now a merely historical 
interest. The former maintained that either in the ovum 
or in the sperm-corpuscle the whole future organism 
