DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 261 



1. The vertebrate lymphatic system is a direct derivative of 

 the embryonal venous system. 



2. At one or more definite points along the course of the em- 

 bryonal veins lymphatic sacs or 'buds' arise from which, by a 

 process of continuous 'sprouting' of the 'Ijrmphatic endothelium/ 

 the lymphatic vessels proper develop and successively invade 

 the embryonal body in a centrifugal direction. From the tips 

 of these lymphatic channels a set of blindly ending closed 'sprouts' 

 are derived which represent the lymphatic capillaries. 



This view involves the acceptance of the theory of 'specificity' 

 of the embryonal vascular endothelium, so that the endothelium 

 lining the early venous channels becomes endowed, by virtue of 

 its 'specific' character, with the faculty of creating, by a con- 

 tinuous centrifugally directed process of sprouting and exten- 

 sion, a system of lymphatic channels which progressively invade 

 the tissues of the embryo from the center toward the periphery. 

 This view may, for the sake of brevity, be defined as the "cen- 

 trifugal theory of lymphatic development." 



I became convinced early in the course of my investigations 

 that Ranvier's view of a venous origin of the lymphatic system, 

 as modified by the advocates of the 'centrifugal theory,' was 

 not supported by actual and valid evidence. At that time I 

 pointed out the futility of attempting the solution of the lym- 

 phatic problem by injecting already connected and continuous 

 portions of the embryonal lymphatic channel system. The value 

 of the objections then raised against the sweeping conclusions 

 drawn from the partial and incomplete evidence furnished by 

 injections has since then been abundantly and clearly demon- 

 strated. 



Injections of the embryonal lymphatics yield easily procured 

 and very beautiful topographical preparations illustrating the 

 extent to which, in the individual stages, the independently de- 

 veloped lymphatic anlages have united into an injectible system 

 of connected channels. The method does not in any way touch 

 the genesis of the lymphatic anlages from which these channels 

 arose. Injections, moreover, do not afford the slightest clue as 

 to the presence or absence of mesenchymal intercellular tissue 



