1 



Fig. 1 Transverse section of an embryo with one pair of intersomitic clefts. 

 Columbia collection No. 554, slide 4, row 5, section 8, X 300. Reduced j. 



1 Ectoderm 5 Hyperentedermal mesenchyme 



2 Mesoderm 6 Mesostroma 



3 Entoderm 7 Coelom 



4 Hypectodermal mesenchyme 8 Neural plate 



Figs. 2 to 4 Dorsal view of a model of the mesoderm and mesenchyme of 

 an embryo of two somites. Columbia collection No. 539, X 300. Reduced \. 

 The mesoderm sensu slricto is white, the mesenchyme green. 



1 First intersomitic cleft 



2 Second intersomitic cleft 



3 From this point craniad the meso- 



derm is continuous across the me- 

 dian line 



From this point caudad the meso- 

 derm is continuous across the me- 

 dian line 



64 



