298 REPRODITTION IN ANIMALS 



bodies. In a typical example (Fig. 70) the nucleus is 

 a highly compact body in the head of the tadpole-shaped 

 cell, while the remaining protoplasm is drawn out to 

 form a slender thread that serves as a locomotor organ. 

 Each sperm is thus a cell whose form fits it for active 

 swimming movements, enabling it to come in contact 

 with an egg in the process of fertilization. 



Eggs, with few exceptions, are spherical cells, without 

 the power of movement, and contain food material, 

 usually in the form of yolk. Where the young animal 

 early shifts for itself, as in the case of the sea urchin, or 



Fig. 70. — Egj? and S{x;rm ( 'OlLs. Smaller .sperm of .same mag- 

 nification as egg, in which some of the food yolk is shown. 



where it draws its food supply from the body of the 

 parent, as in the rabbit, the amount of yolk is small and 

 the egg scarcely measures over one or two hundredths of 

 an inch in diameter. Nevertheless such ova are many 

 hundreds of thousands of times larger than the sperms of 

 the same species. At the opposite end of the series the 

 amount of yolk is enormous. In a Japanese shark, for 

 example, the egg is over eight inches in diameter, in the 

 ostrich it is over three inches, in the domestic fowl one 

 inch, and in many other fishes, lizards, and turtles the 

 egg cells are only slightly smaller. 



Accessory Sex Organs in the higher animals are those 

 by means of which sperms and eggs are brought together. 



