Development of the Mammal 291 



tached to the subzonal layer at one place (Fig. 

 89). This hollow sphere, characteristic of 



cc 



cc 



FIG. 88. FIG. 89. 



FIG. 8S. A RABBIT'S OVUM SEVENTY HOURS AFTER 



COPULATION, TAKEN FROM THE LOWER END OF THE OVI- 

 DUCT JUST BEFORE ENTERING THE UTERUS, AND SHOWING 

 THE CONDITION AT THE CLOSE OF SEGMENTATION. (After 

 Van Beneden.) X2OO. 



FIG. 89. A RABBIT'S OVUM SEVENTY-FIVE HOURS AFTER 



COPULATION, TAKEN FROM THE UTERUS, AND SHOWING THE 

 FIRST STAGE IN THE FORMATION OF THE BLASTODERMIC 



VESICLE. (After Van Beneden.) X2OO. 



CC, outer layer of cells. CD, inner mass of cells. 

 of blastodermic vesicle. 



cavity 



mammals, is known as the blastodermic vesicle. 

 It is usually said that the subzonal layer cor- 

 responds to the ectoderm, the inner mass to 

 the entoderm. 



The changes just described have taken place 

 during the passage of the ovum through the 

 Fallopian tube towards the uterus. How long 

 this period may be in man is not known ; Minot 



