86 



TEXT-BOOK OF EMBRYOLOGY. 



FIG. 54. Four stages in cleavage of the ovum of the mouse. Sobotta. Small cell marked with 



x is the polar body. 



FIG. 55. Four stages in the development of the bat. van Beneden. 



a, Section of morula; b, section of later stage of morula, showing differentiation of outer layer of 

 cells; c, section of still later stage, showing vacuolization of central cells; d, section showing 

 outer layer (trop'hoderm) and inner cell mass. 



