92 



PHYSIOLOGY 



(Fig. 55). All of these cells have a marked affinity for 

 oxygen. Those on the inside therefore push their way 



Fig. 55.— a, egg cell; b,c,d,e,f, successive stages of division. (From Sedgwick and 

 Wilson.) 



to the outside toward the oxygen. In this way they 



become arranged in a hollow sphere called a blastula 

 (Fig. 56). As these cells have 

 their affinity for oxygen satisfied, 

 some of them develop a new affin- 

 ity. They are attracted toward 

 other cells and move inward until 

 a pear-shaped structure called a 

 gastrula (Fig. 57) is formed, 

 which has a wall composed of two 



layers of cells and an opening leading from the central 



cavity to the outside. 



Differentiation of Cells.— The two layers of cells are 



Fig. 56. — Diagram of a 

 blastula. 



