252 



GENERAL EMBRYOLOGY 



"The history of the fourth quartet is peculiar ..... 

 The posterior cell 4d is the mesentoblast, from which the ali- 

 mentary canal and a portion of the mesoderm arise. The 

 other three cells of the fourth quartet, 4a, 4b, 4c, do not divide 

 as long as their history can be traced. They, however, break 

 up into a large number of homogeneous yolk spheres which are 

 absorbed by the endoderm cells. The large nuclei of these 

 three cells can be traced until the alimentary canal is partly 

 formed. 



TABLE OF THE CELL LINEAGE OF PLANOCERA 

 Condensed from Surface. 



la 11 

 | ia- i 



la 



v la 2 

 A (left) 



2a 



AB 



B (ant.) 



Zygotc 



C (rt.) 



CD 



D (post.) 



