so 



GENERAL PRINCIPLES OF ZG0L00 7. 



(figp. 33, p, 34), aiiid 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 spermatozoa 

 the smallest, cells of the animal body. 



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

 formed in the ovarj^ varies in size according 

 to the aiiimal grouji : in case of the micro- 

 scof)ic Gastrotriclia 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 difl^erences in size are 

 —f 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 



"5b5 



i N> 4 



FlO. 34. 



Fig. a5. 



Fio. 34.— Ovarian tube of an insect, Vancsa^n xirtirir. a, furmative cell; h, follicular 

 epitlieliuTn; c, nutritive cells; li, etJt^-cells; /, fibrous covering esteuding out into 

 tiic terminal fibres ((/). (After M'aldeycr.) 



Fig. 35.— Egg-cell of Sfronyiyfocciifrodis lividwt. 



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 



