60 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



continuity of lumen with those of the yolk-sac, for it is incon- 

 ceivable that in such circumstances, a beating heart could fail 

 to effect a circulation. 



The investigations of Stockard ('15) supplement and coin- 

 cide with those of Hahn, Miller and McWhorter, Reagan, and 

 Loeb in a most decisive manner. Stockard has shown that, not 

 only do anesthetics arrest the development of the intra-embryonic 

 blood vessels in the embryos of Fundulus, at an early ontogenetic 

 stage, but in such a manner that no doubt can now exist that, 

 under normal conditions, these vessels are formed in situ by a 

 concrescence of independent and discontinuous anlagen, and 

 that their endothelium is derived directly from intra-embryonic 

 mesenchymal cells. It is interesting to note in this connection 

 that Wenkebach ('86) had already observed in the body and 

 yolk-sac of the living fish embryo (Belone longirostris), that 

 mesenchymal cells play an important rdle in the formation of 

 vessels and sprouts. In their general features the observations 

 of Wenckebach have been confirmed by Raffaele ('92). 



It is thus seen that experimentation bears out the observations 

 made upon fixed and living material, that the intra-embryonic 

 blood-vascular channels do not grow into the embryo from the 

 yolk-sac, but are formed in situ by a concrescence of independent 

 and discontinuous anlagen, whose endothelium is formed from 

 intra-embryonic mesenchymal cells. 



The vascular plexus formed in the extra-embryonic area of 

 the vertebrate embryo, is as we know, at first represented by 

 discontinuous, independent and circumscribed anlagen, the 

 cells of which possess a local origin. Clefts or spaces, the future 

 lumina of the plexus, soon make their appearance in a discontinu- 

 ous manner amongst the cells of these anlagen, and it is by a 

 concrescence of these vascular spaces that a continuous system 

 of vascular lumina is finally formed. The cells which con- 

 stitute the walls of these vascular spaces become transformed 

 into the endothelium and, when blood-islands are present, the 

 more centrally situated cells form the primary blood cells. It 

 is interesting to note in this connection that McWhorter and 

 Whipple ('12) have recently been able to demonstrate and record 



