()2 



FRANKLIN P. REAGAN 



plete isolation of the urivascularized tissue, and a means of 

 obtaining accurate knowledge of the conditions in the outlying 

 tissue. Given these, the method becomes a critical one. It 

 should satisfy all reasonable demands of experimental proof. 

 To say the very least, the endothelium which can develop in 

 the isolated tissue under such conditions cannot have grown 

 in from an outside source. 



Whether the endothelium which undoubtedly does form under 

 these conditions represents in a faithful manner the essentials 

 of its normal development is not for me to say. All that can 

 be claimed is that my figures show conditions which were actu- 

 ally obtained. Further study alone can determine what sig- 

 nificance is to be attached to these conditions. 



Explanation of abbreviations used in table: m.h.-e. refers to my modification 

 of Mann's methyl blue-eosin stain, Fe.h. indicates iron haematoxylin; Bx.c. 

 indicates borax carmine. Two plus-marks indicate that the extraembryonic 

 bLastoderm is shown in illustration. 



