ONTOGENESIS 



227 



tiles, birds, and the lowest mammals, we return once 

 more to holoblastic eggs as we find them in mammals. 



FIG. 90. FIG. 91. 



FIG. 90. Ovum of the bat, showing vacuolation of the segmented egg to 



form the blastodermic cavity. X 500. (Van Beneden.) 



FIG. 91. Ovum of the bat, showing vacuolation of the segmented egg to form the 

 blastodermic cavity. X 600. (Van Beneden.) 



These begin development by an almost equal segmen- 

 tation resulting in the formation of a morula, composed 

 of cells of fairly uniform size. When the morula stage 

 has been completed, vacuoles begin to appear in some 

 of the central cells, become larger, coalesce, and give 

 origin to an irregular fissure-like space which is the 



FIG. 92. Ovum of bat; differentiation of embryonic button. (Fan Beneden.) 



segmentation cavity, or lecithocele, and brings the ovum 

 to the stage of the blastodermic vesicle. Though not 



