1 



Fig. 1 Transverse section of an embryo with one pair iiT iutersoniitii' clcfls. 

 Columbia eollection No. 554, slide 4, row 5, section 8, X 300. lleduced {. 



1 Ectoderm 5 Hyperentedermal mesenchyme 



2 Mesoderm 6 Mesostroma 



3 Entoderm 7 Coelom 



4 Hypectodermal me.scnchyme 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 3(K). Reduced 2- 

 The mesoderm sensu slricto is white, tlie mesenchyme green. 



1 First intersomitic cleft 4 From this point caudud the iiic.>;ii- 



2 Second intersomitic cleft derm is cordinu<ius aci-oss the inc- 



3 From this point craniad the meso- ilian line 



derm is <'onHniious across the me- 

 dian line 



54 



