INTRODUCTION. 



21 



this stage of development, there is a cellular sac, containing fluid, 

 with a reinforcement of its wall at the region occupied by the primi- 

 tive entoderm (Fig. 3). 



FIG. 



Segmented egg of Petromyzon Planeri : Surface view of the collection of cells. The nuclei 



are invisible. (Kupffer.) 



Subsequent to these events a third layer of cells becomes inter- 

 posed between the primitive ectoderm and entoderm. Most of its 



FIG. 3. 



Ovum of rabbit : a, primitive ectoderm in section ; 6, primitive ectoderm, surface view ; 

 c, primitive entoderm ; d, dividing cell of the ectoderm, (van Beneden.) 



cells are derived from those of the primitive ectoderm, but the 



