140 OUTLINES OF EVOLUTIONARY BIOLOGY 



sh.m. 



FIG. 70. Diagram of the Structure 

 Bird's Egg. 



a.c., air chamber; alb., albumen; ch., chalazte, 

 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. TO., vitelline membrane ;y k., "yolk." 



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 



n \" f? , provided with a lateral 



undulating membrane. 

 A typical ovum is a 

 relatively large, spheri- 

 cal cell with a con- 

 spicuous nucleus (mi.) 

 surrounded by a large 

 quantity of cytoplasm 

 (cyt.), and the whole 

 enclosed in a delicate 

 vitelline membrane 

 (v.m.). The actual size 

 of the ovum depends 

 almost entirely on the 

 amount of food material 

 (deutoplasm or yolk), 



which is stored up in the cytoplasm in the form of granules (?/.//.). 



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



small, and the egg is only about alo^h 



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 " (fj.d.), containing the 



nucleus (nu.), which lies within the 



vitelline membrane (r.m.) on the top 



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



proper is entirely enclosed in accessory 



structures the " white " or albumen 



(nib.) 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 ^ a 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 Bischoff.) 



MO, spermatozoa which have 

 penetrated the zona radiata ; 

 N, nucleus; NU, nucleolus; 

 Z, zona radiata. 



