RELATION OF LYMPHATIC TO BLOOD- VASCULAR SYSTEM 47 



ical concept involved in this term seems, to judge from a recent 

 publication, 33 to have been difficult to acquire. I am glad to be 

 able to make myself clear by reference to fig. 17, where the mutual 

 relation of the two vascular lumina is evident without further 

 description, and to the numerous detailed illustrations on a larger 

 scale of magnification which accompany the account of the devel- 

 opment of the thoracic ducts in Part II of this communication. 



By far the larger number of the early lymphatic channels are 

 the product of fusion of these " extra-intimal " spaces, and hence 

 closely follow the veins of their respective regions. Subsequently, 

 with the development of a venous adventitia, this relationship 

 is somewhat altered in case of those veins which are included in 

 the permanent venous organization. The close relation existing, 

 however, throughout life between these veins and the accompany- 

 ing lymphatics is based on this intimate primitive association of 

 their respective anlages. 



On the other hand, the extra-intimal position of the earliest 

 lymphatic spaces furnishes the explanation of another relation 

 manifested between the developing systemic lymphatic channels 

 and those embryonic veins which in course of normal venous devel- 

 opment are destined to undergo reduction and finally complete 

 suppression, when the primitive bilateral and symmetrical venous 

 system of the earlier embryonic stages shifts to the dextral as- 

 symmetrical type of the main adult axial channels. In these 

 circumstances the systemic lymphatic vessel associated with the 

 temporary embryonic vein experiences, apparently through the 

 shrinkage of the latter, an impetus to its own more extensive de- 

 velopment, so that it comes to occupy in general topographically 

 the space filled by the vein in the earlier stages. 



Thus the embryonic period which marks the normal ontogenetic 

 swing of the main venous line to the right through the secondary 

 sinistro-dextral iliac, hemiazygos and brachiocephalic cross anas- 

 tomoses, sees the simultaneous increase in the corresponding lym- 

 phatic channels of the left side, which topographically replace the 

 abandoned left embryonic venous pathways of the earlier and sym- 



33 Sabin: Anat. Rec., vol. ii, 1908, p. 50. 



