THE GERM-CELLS 



35 



(Crusters, Insects) ; (3) polar yolk, a large proportion of 

 yolk, accumulated chiefly in the lower half of the egg, as in 

 Amphibians (Frog) ; and finally (4) predominant yolk, 



A 



B 



-ax 



D 



Fig. 24, — Ovum and Disposition 

 OF Yolk. (After Thomson.) 



A, ovum with diffuse yolk (sponge) ; 

 B, ovum with central yolk (cray- 

 fish) ; C, ovum with polar yolk 

 (frog) ; D, ovum with predomin- 

 ant yolk (bird). 



Fig. 25. — Spermatozoon. 

 (After Wilson.) 



{From Weismann, " The 

 Evolution Theory.") 



sp, point ; 77, nucleus ; c, 

 centrosome ; m, middle- 

 piece ; aXy tail. 



where nearly the whole egg is taken up by the yolk except 

 a tiny part at the upper pole of it, as in Fishes, Reptiles, 

 and Birds. 



