GERM-CELLS. 45 



In many animals it is provided with a sheath or mem- 

 brane of its own; in others it remains naked, and in that 

 case frequently displays the remarkable movements of 

 protoplasm. The germ-cells of different classes of ani- 

 mals 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 per- 

 forms 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 modi- 

 fied 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 



