80 



GENERAL PRINCIPLES OF ZOOLOGY. 



(figs. 33, p, 34), and thus in many groups of animals the sexual 

 organs bear the character of branched 

 glands; for this reason one speaks as often 

 of sexual glands as of sexual organs (fig. 34). 

 The male and female cells, the specific ele- 

 ments of the germinal epithelia and of the 

 sexual glands, differ in the fact that the eggs 

 are generally the largest, the spermatczoa 

 the smallest, cells of the animal body. 



Egg-cell. The egg-cell (fig. 35) as it is 

 formed in the ovary varies in size according 

 to the animal group: in case of the micro- 

 scopic Gastrotricha 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 ; however, only the yolk of the 

 bird's egg is the egg-cell, the white of the 

 egg and the shell are structures which are 

 formed outside of the ovary in the oviduct. 

 These remarkable differences in size are 

 caused 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 



FIG. 34. 



FIG. 35. 



a, formative cell; 7), follicular 



epithelium; c, nutritive cells; d,J5gg-cells; /, fibrous covering extending out into 



aid 



FIG. 34. Ovarian tube of an insect, Vanessa urticce. 



epithelium; c, nutritive cells; d, egg-cells; 



the terminal fibres (g). (After Waldeyer.) 

 FIG. 35. Egg-cell of Stronyylocentrotus Uvidus. 



briefly called yolk). The function of the deutoplasm is to nourish 

 the embryo during development, and hence consists of substances 

 rich in fat and proteid, arranged in spherical oil-drops, or in fine 



