Metazoa Ran a. 



331 



its roof becomes, therefore, hypoblast. At the same time 

 between the epiblast and hypoblast above, and the epiblast 

 and yoke-cells on the other side, several layers of cells 

 are formed the mesoblast Lastly, the floor of the mesen- 



FIG. 187. -STAGES IN THE DIVISION OF THK FROG'S OVUM. (Ecker.) 

 1248 



FIG. 188. DIAGRAMMATIC LONGITUDINAL SECTION 

 OF EMBRYO. (Ow en.) 



teron is formed partly of hypoblast cells derived from the 

 superficial cells of the yolk, and partly of an ingrowth of 

 epiblast. The only difference, therefore, at this stage be- 

 tween the embryo of Amphioxus and that of Rana lies in 

 the possession by the 

 latter of a quantity 

 of food-yolk (after- 

 wards absorbed), the 

 presence of which 

 to a certain extent 

 alters the shape of 

 the embryo and 

 renders the 

 in its development 

 rather more difficult to follow. 



It is unnecessary to follow the various changes in the 

 development of the embryo in detail here ; we may rather 

 conclude this section by a brief consideration of the mode 

 of origin of the sstenth^?&^^vhich shows the 



Sta CT eS "' neuro-vertebral axis; a, i, layers of the abdominal 

 wall ; r, heart ; hv, yolk. 



