264 GEO. S. HUNTINGTON 



to form the proximal portion of the thoracic duct. But examina- 

 tion of the uninjected and uninjectible field caudal to the tho- 

 racic duct approach in figure 1 shows that the apparent centrifu- 

 gal downward growth of the thoracic duct in figure 2 is due to 

 a process of accretion in the reverse centripetal direction of the 

 previously separate and independent tissue spaces (10) into a 

 connected channel which has joined the thoracic duct approach 

 and accounts for the apparent caudal prolongation of the same. 

 In figure 3 (16 nmi.) the further caudal extension of the thoracic 

 duct (11), as indicated by the injection, is again in the same 

 way not due to a continuous downward growth, but to the suc- 

 cessive accretions to its length obtained by fusion with the distal 

 independent mesenchymal spaces of the previous stages (10). 



Taken by themselves and judged solely by the. results of the 

 injection test, these three embryos might be regarded as afford- 

 ing proof of the progressive sprouting or growth of a lymphatic 

 channel from the veins toward the periphery. Judged by the 

 examination of the non-injectible area, beyond the limits of com- 

 plete injection, this entire theory collapses, if it can be proved 

 that in this non-injectible area the earlier lymphatic anlages 

 exist, which, by successive fusion with eacli other and with the 

 central portions of the system, produce the lymphatic progres- 

 sion demonstrated in successive stages by the injection. The 

 proof of the existence of these true lymphatic anlages, constant 

 in occurrence, uniform in location and relations to surrounding 

 structures, and as easily and definitely diagnosed by an experi- 

 enced eye as the more striking haemal vascular channels, has, 

 I think, been abundantly and conclusively afforded in the last 

 few years by a number of observations (5, 6, 7, 8, 9, 10, 11, 12, 

 13, 14, 15). 



These investigations cover not only observations on mamma- 

 lian embryos but also researches on the development of the 

 lymphatic system in the birds and reptiles. 



II. The second main theory of lymphatic development regards 

 the lymphatic vessels of the adult vertebrate as the result of 

 successive fusions of embryonal intercellular mesenchymal spaces. 



