GENERAL PRINCIPLES OF ZOOLOGY. 



Egg-cell. The cgg-ccll (Fig. 33) as it is formed 

 in the ovary has various sizes according to the animal 



group: in case of the microscopically 

 small GastrotricJia, it is less than 0.04 

 mm., in man about 0.2 mm., in the 

 frog several millimetres, and in the 

 large birds often several inches ; how- 

 ever, only the so-called "yellow" 

 of the egg is . to be regarded as 

 the egg-cell, while the white of the 

 egg and the shell are structures 

 which are formed outside of the 

 ovary in the oviduct. These re- 

 markable differences in size are con- 

 ditioned less by the quantity of the 

 peculiar cell-substance, protoplasm 

 (formative or primary yolk), than by 

 the accumulation of deutoplasm (food 

 or accessory yolk, also briefly called 

 yolk). The function of the acces- 

 f sory yolk is to nourish the embryo 

 during development, and hence con- 

 sists of substances rich in fat and 

 proteid, arranged in spherical oil- 

 drops, or in fine granules or polyg- 



FIG. 32. Fie;. 33. 



FIG. 32. Oviduct of an insect, I'anessa urticte. a, formative cell ; b, follicular epithe- 

 lium ; c, nutritive cells; d, egg-cells ; /, fibrous covering extending out into the ter- 

 minal fibres (g). (After Waldeyer.) 



FIG. 33. Egg-cell of Strongylocentrotus iividus. 



onal bodies, the yolk-particles. It is present in so much 



