MODEL 16 



Second division of mesentoblast, resulting in the formation of two meso- 

 blasts and two entoblasts, which are concealed under the rim of the plate of 

 ectoblasts, further divisions of ectoblast cell 



MODEL 17 



Continued spreading of ectoblasts over the surface of the egg. Their 

 origin from the three quartets is indicated in the model by colors: first 

 quartet, red; second, blue; third, uncolored. 

 [22] 46 



