GENERAL HISTOLOGY 



71 



sexual glands, differ from each other in the fact that the eggs are generally 

 the largest, the spermatozoa the smallest, cells of the animal body. 



Egg-cell. — The egg-cell or oocyte (fig. 36) as it 

 is formed in the ovarj' varies in size according to 

 the animal group: in case of the microscopic 

 Gastrotricha it is less than 0.04 mm., in man about 

 0.2 mm., in the frog several millimetres, and in 

 large birds often several inches; however, only the 

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

 the shell are structures formed outside of the 

 ovary in the oviduct. These remarkable differ- 

 ences in size are caused less by the quantity of 

 the peculiar cell-substance, protoplasm (primary 

 yolk), than by the accumulation of deiitoplasm 

 (food or accessory yolk, also briefly called yolk). 

 The deutoplasm is to nourish the embryo during 

 development, and hence consists of substances 

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

 in fine granules or polygonal bodies, the yolk- 

 granules. Its quantity, and therefore the size of 

 f the egg, is in part proportional to the length of 

 time which the egg is cut off from any other 

 supply of nourishment. In general we find the 

 largest eggs in the case of the highly organized 

 oviparous animals, where a long development 



Wi 



f—l 



l\ 



Fifi. 35 

 YiG. 35. — Ovarian tube of an insect, Vanessa la-tkm. 



a, formative cell; 6, follicular 

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

 terminal fibres (?) fafter Waldeyer). 



Fig. 36. — Immature egg-ceU. of Strongyloceiilrotus lividus. 



inside of the shell is necessary to lay the foundation of the manifold 

 organs. Besides the protoplasm and deutoplasm, a cell nucleus or 



