140 OUTLINES OF EVOLUTIONAKY BIOLOGY 



ch. 



ch. 



the active locomotor organ, by the movements of which the 

 spermatozoon swims about ; it contains an axial filament (ax.) 



and may or may not be 

 provided with a "lateral 

 undulating membrane. 

 A typical ovum is a 

 relatively large, spheri- 

 cal cell with a con- 

 spicuous nucleus (nu.) 

 surrounded by a large 

 quantity of cytoplasm 

 (cyt.), and the whole 

 enclosed in a delicate 

 vitelline membrane 

 (v.m.). The actual size 



a.c. 



alb. 



v.m. 



FIG. 70. Diagram of the Structure of a 



Bird's Egg. 

 a.c., air chamber; alb., albumen; ch., chalazse, of the OVUm depends 



twisted cords of dense albumen which serve to 



keep the " yolk " in position ; g.d., germinal 



disk; nu., nucleus; sh., shell; sh.m., shell 



membrane; v.m., vitelline membrane ; yk., "yolk." 



/^n 

 On 



NU 



amount of food material 

 (deutoplasm or yolk), 



which is stored up in the cytoplasm in the form of granules (y .g.). 



In Amphioxus (Fig. 13, 1) the amount of food material is very 



small, and the egg is only about ^Jo* n 



inch in diameter. 



An extreme contrast to the egg of 



Amphioxus is seen in that of a bird 



(Fig. 70), where the amount of yolk 



is enormously large and the active 



protoplasm is confined to a minute 



" germinal disk " (g.d.), containing the 



nucleus (nu.), which lies within the 



vitelline membrane (v.m.) on the top M0/ 



of the "yolk" (yk.), while the ovum 



proper is entirely enclosed in accessory 



structures the " white " or albumen 



(alb.) and the shell (sh.), with its lining 



membrane (sh.m.). 



The mammalian ovum, on the other 



hand, is, like that of Amphioxus, very 



minute, in the rabbit (Fig. 71) again only about ^th inch in 



diameter. It" is enclosed in an envelope known as the zona 



radiata (Z), which lies outside the vitelline membrane, and it 



IG ' 



" Vertebrate Embry- 

 ology," after Bischoif.) 



MO, spermatozoa which have 

 penetrated the zona radiata ; 

 N, nucleus; NU, nucleolus; 

 Z, zona radiata. 



