Fig. 20 Transverse section from embryo in which the somites are forming, 

 but no complete intersomitic cleft has appeared. Columbia Collection No. 550, 

 slide 3, row 4, section 6, X 300. Reduced . 



1 Ectoderm 



2 Mesoderm 



3 Entoderm 



4 Hyperentodermal mesenchyme and 



incipient blood islands 



5 Coelom 



75 



