504 



STRUCTURE OF OVUM 



CHAP. 



Structure of the Ovum. The striking general resem- 

 blance between the cells of the higher animals and 

 entire unicellular organisms has been commented on as 

 a very remarkable fact : there is another equally signifi- 

 cant circumstance 



Head 



nuls 



to which we must 

 give our attention. 



All the higher 

 animals begin life 

 as an egg, which 

 is either passed 

 out of the body of 

 the parent, as 

 such, as in earth- 

 worms, crayfishes, 

 frogs, birds, &c. 

 (oviparous forms) , or 

 undergoes develop- 

 ment in the body 

 of the parent, as 

 in some dogfishes 

 (p. 470) and nearly 

 all mammals (vivi- 

 parous forms) . 



The structure of 

 an egg is, in essen- 

 tial respects, the 

 same in all animals 

 from the highest 



to the lowest (compare p. 195). It consists (Fig. 148) of 

 a more or less globular mass of protoplasm (cyt}, spoken 

 of as the vitellus, in which are deposited proteinaceous 

 particles known as yolk-granules (y.g). \Vithin the proto- 

 plasm is a large nucleus containing chromatin as well as 

 one or more nucleoli (nu, nuls) which are often known 



FIG. 148. Diagram of typical sperm and ovum, the 



former much more magnified than the latter. 

 ax. axial filament ; e.g. chromatin granules ; cs 

 centrosome ; cyt. cytoplasm ; m.p. middle piece 

 n.m. nuclear membrane ; nu. nucleus ; nuls 

 nucleolus ; v.m. vitelline membrane ; y.g. yolk 

 granules. (From Dendy's Evolutionary Biology.) 



