14 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



(a) With the above defined independently formed systemic 

 lymphatic channels of the entire body. 



(6) Secondary connections with the venous system, re-entering 

 the same at one or more typical and constant points, and thus 

 forming the link which finally unites the venous and the lymphatic 

 systems, developed independently of each other. 



The above are, briefly summarized, the views of lymphatic 

 development based on recent observations. 



It will be seen, as previously stated, that opinions still differ 

 as to the origin of the first lymphatic anlages and their subse- 

 quent method of growth, and as to the genetic derivation of the 

 lymphatic vascular endothelium. 



The following two main questions are therefore still to be 

 definitely answered: 



(1) Is the adult mammalian lymphatic system the result of 

 continuous and uninterrupted growth from one or more central 

 points toward the periphery, or is it genetically a channel system, 

 developed on the same lines as the primary blood vascular sys- 

 tem, by the confluence of a number of originally separate and 

 independent anlages? 



(2) Is the lymphatic vascular endothelium of the mammal 

 derived from pre-existing haemal vascular endothelium, or is it 

 the result of independent modification of mesodermal cells? 



It now remains to answer definitely these questions, and, on 

 the evidence of sufficiently extensive material and careful obser- 

 vations, to clear the field of theoretical considerations, and to 

 establish, as far as possible, by sound methods and on a broad 

 basis, the genesis of the mammalian lymphatic system as a whole. 

 It is evident that an interpretation, which assumes to fulfill these 

 conditions, must be capable of accurately standing the test of 

 both ontogenetic and phylogenetic consistency. 



Believing, as I do, and have, since my first expression of opinion 

 on the subject, that the principles embodied in the genetic inter- 

 pretation of mammalian lymphatic development above outlined 

 (VI) are correct, I have undertaken to establish their truth by a 

 detailed critical study of the lymphatic system, both in the 

 adult and in the embryo, in one mammalian form, the Domestic 



