GERM-CELLS. 



45 



In manyanlmals 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 s), which 

 consist of a pointed, elliptic, or 

 occasionally of a hook-shaped, 

 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 



