206 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



used later as food by the growing embryo. It appears in the 

 shape of small disks, plates, or globules embedded in the proto- 

 plasm. In color it is usually white, but in birds (Fig. 212) two 

 kinds of yolk, white and yellow, are arranged in a complicated 

 manner. The amount and distribution of this deutoplasm exer- 

 cise an important influence upon the early phases of develop- 

 ment, while the size of the egg is directly dependent upon the 

 amount of this substance. 



In the higher mammals the deutoplasm is scanty in amount, 

 and is regularly distributed throughout the protoplasm (ale- 



FlG. 212. Diagram of hen's egg, from Hertwig after Balfour. ac/i, air 

 chamber; bl, blastoderm; t/i/, chalaza ; ism^ inner shell membrane; s, shell; $i, 

 outer shell membrane ; z'/, vitelline membrane ; w, white ; wy, white yolk ; JT, inner 

 layer of white; }'}', yellow yolk. 



cithal). In the amphibia and in Petromyzon the yolk is much 

 more abundant, and the eggs are consequently larger in size. 

 It is still distributed throughout the whole of the egg ; but a 

 marked polarity is visible, one pole of the egg containing the 

 bulk of the protoplasm, while the other is as strongly deutoplas- 

 mic (telolecithal). In selachians, reptiles, birds, and mono- 

 tremes this polarity is still more marked ; while in many teleosts 

 the extreme is reached, for here protoplasmic and deutoplasmic 



