140 OUTLINES OF EVOLUTIONARY BIOLOGY 



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 



sh.m,. 



a.c. 



yh. 



NU 



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 Q l 1T ,^ a f nfii-aliT ^ i flir, 



keep the " yolk " in position ; g.d., germinal al11 6l J ( 



disk; nu., nucleus; sh., shell; sh.m., shell amount of food material 



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



(deutoplasm or yolk), 



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

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

 small, and the egg is only about ^io^ 1 

 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 " (c/.d.), containing the 

 nucleus (nu.), which lies within the 

 vitelline membrane (v.m.) on the top 

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

 proper is entirely enclosed in accessory 

 structures the " white " or albumen 

 (<dl>.) 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 



FIG. 71. Ovum of a Babbit, 

 X 200. (From Marshall's 

 " Vertebrate Embry- 

 ology," after BischofP.) 



MO, spermatozoa which have 

 penetrated the zona radiata ; 

 N, nucleus; NU, nucleolus; 

 Z, zona radiata. 



