62 



THE CELL 



The earlier generations of these cells are morphologically alike 

 and are known as Uastomeres. Soon, however, these cells become 

 spread out and at the same time differentiated into two primary 



ZONA 



PE ULUCIDA 



POLAR 

 GLOBULES 



A 



SEGMENTA- 

 TION CAVITY. 



Fig. 13. — Segmentation of the Ovum. (From Gerrish, after van Beneden.) 

 a, Two-cell stage resulting from first division of fertilized ovum; h, four-cell stage; 

 c, d, e, later stages. A, Differentiation into inner and outer cells; B, Formation of 

 segmentation cavity; C, Embryonic vesicle, showing two primary germ layers. Outer 

 cells, ectoderm; inner cells, entoderm. 



