THE FORMATION OF THE LAYERS IN AMPHIOXOS. 321 
Text-fig. 7. 
7 . 
Median sagittal sections of the gastmlas of tlie Stm-geon, ChiniiBra 
and the Shark, s. True segmentation cavity, s'. Inner part 
of archenteric cavity sometimes formed by cleavage of the 
endoderm. (After Bashford Deane.) 
