EARLIEST BLOOD-VESSELS IN MAN 465 



Extension within the limit of the areas covered by the meso- 

 thelium is achieved by confluence of the detached portions of the 

 coelom, or union of the cords; the result is a net comprising the 

 various vascular units. Extension into the chorion, where the 

 mesothelial layer is absent in the early stages, appears to be by 

 direct centrifugal growth of the angioblast cords, without the 

 addition of new elements from the surrounding mesenchyma. 



The possibility that similar, but later, ingrowths from the meso- 

 thelium of the intra-embryonic coelom may give rise to intra-embry- 

 onic vessels, should be borne in mind in the study of such vessels, 

 whether haemal or lymphatic. 



