EARLIEST BLOOD-VESSELS IN MAX 463 



processes, though it is often less than that of the cord nuclei. 

 The cords tend to form nets by anastomosis of larger mesh than 

 the mesenchymal net, and angiocysts by vacuolization wherever 

 space is given. They are usually sharply defined from the sur- 

 rounding tissue, and may show an extra-intimal space. They 

 must necessarily be extremely hard to recognize in dense mesen- 

 chyma, though easy to trace in perfectly prepared series of looser 

 tissue. 



A second possibility to be considered in attempting to explain, 

 on the basis of the specificity of endothelium, the presence of 

 vessels on the injured side of the operated em.bryos is advanced in 

 this present paper. If the earliest human vessels arise from the 

 mesothelium lining a portion of the extra-embryonic coelom, by 

 multiple anlages which later fuse; and if the intra-embryonic 

 coelom, developed later, is also lined by mesothelium, the oppor- 

 tunity is perhaps offered for similar, but later, anlages for intra- 

 embryonic vessels. The instance cited by Huntington of a coel- 

 omic opening into the spaces of the early lymphatic plexus should 

 be borne in mine. I am well aware that the yolk-sac blood-islands 

 are not proven to arise from mesothelial anlages, that in avian em- 

 bryos they are even said to be present before the formation of the 

 coelom in the area vasculosa, a fact that would point to some tissue 

 other than mesothelium (perhaps a premesothelial stage of nieso- 

 denn) as that from which endothelium is derived; yet certain 

 connections between the vessels of the operated embryos above 

 referred to and the mesothelial wall of the coelom, shown in many 

 of the drawings and seen by me in one of Miller's preparations, 

 seem to me to make this possibility at least worthy of consideration 

 in future study. It is easily conceivable that such separate 

 anlages might arise under abnomial conditions in positions where 

 they are normally absent; or on the other hand it may be that 

 multiple anlages of intra-embryonic vessels in close relation to the 

 coelom are normal, and that the net figured by me in this pcsi- 

 tion is the result of their confluence. Yet the finding of no 

 separate angioblast cords in advance of the general net in this 

 specimen, especially at the younger caudal end, would militate 

 against this latter proposition. It is also possible that embryos 



