40 



COMPARATIVE ANATOMY 



some correlation between the size of the egg and the size of the animal. 

 Thus, among birds, the humming-bird's egg is smallest, and the largest 

 were produced by certain extinct relatives of the ostriches, the moas of 

 New Zealand and ^^pyornis of Madagascar. 



G' 'OV 



Fig. 31. — -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.) 



The great size of the eggs of reptiles and birds (Fig. 32) is due mainly 

 to the enormous amount of yolk present. In the comparatively small 

 eggs of amphibians (Fig. 31) and most fishes, the yolk substance is dis- 



Dudeui of Pander 



neck of latebra 



dense albumen 

 Jess dense albumen 

 vitelline membrane 



Fig. 32. — 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.) 



persed throughout the cytoplasm, but there is a tendency — the more 

 marked the greater the quantity of yolk — for the yolk granules to become 

 aggregated at one side of the egg. Thus, a more or less definite polarity 

 of the egg results. The proportion of yolk to cytoplasm is least at the 



