REPRODUCTION 



29 



of reproduction. Otherwise mammalian eggs are of microscopic dimen- 

 sions (0.06 to 0.3 mm. in diameter). The egg (Fig. 29) is covered by a 

 delicate membrane (zona pellucida) external to which may be a cellular 

 membrane (corona radiata), both contributed by the ovary. 



G' 'OV 



Fig. 27. — Amphibian eggs. A, of frog, soon after laying; B, early larva of frog, just 

 before hatching; C, of the salamander, Cryptobranchus allegheniensis. A and C, 

 approximately actual size; B, enlarged. G, gelatinous layer; L, larva; OV, ovum. 

 {A and B, redrawn from Marshall, "Vertebrate Embryology"; C, after A. M. Reese.) 



Fertilization. Development is initiated by the "fertilization" of 

 the egg. A spermatozoon penetrates the egg (impregnation) and the 

 sperm chromatin becomes joined with the chromatin of the egg nucleus. 



nucleus of Pander 



neck of latebra 



white yolk 



less dense albumen 



yellow yolk ^ 



Fig. 28. — Diagram representing a section of a hen's egg cut in a plane including the 

 long axis of the egg and passing through the blastoderm. (From Patten, "Embryology 

 of the Chick"; after Lillie.) 



The "maturation" process through which all germ cells pass reduces 

 their chromatin to approximately half that contained in body cells, 

 so that the union of sperm chromatin and egg chromatin provides the 

 fertilized egg with a nucleus containing the full complement of chromatic 

 bodies (chromosomes) characteristic of all body cells of the animal. 



