DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 303 



which finally result in the substitution of the subclavian for the 

 primitive ulnar vein. 



In part, as above stated, the development of the massive cords 

 of the brachial plexus appears to exert an important mechanical 

 influence on the rearrangement of the lymphatic system. This 

 factor seems to render the early route of the circulation through 

 the primitive ulnar vein and lymphatic, dorsal to the developing 

 plexus, more difficult to maintain, and favors the secondary 

 development of both venous and lymphatic channels along the 

 easier path of the subclavian line, caudal and ventral to the 

 nervous plexus. 



3. Finally, comparison of lymphatic development of reptilian 

 with avian and mammalian embryos shows a remarkable corre- 

 spondence in the basic genetic processes. In reptiles both the 

 jugular lymphsacs and the systemic lymphatic channels are 

 developed along the same lines as in birds and mammals (12, 14). 

 While this general agreement in the fundamental principles gov- 

 erning the development of both the haemal and the lymphatic 

 divisions of the vascular system obtains in all the three amniote 

 classes, the reptilian embryo exhibits certain special characters, 

 when contrasted in detail with the mammal and the bird. These 

 dififerences are founded on the functional and structural adaptions 

 of the genetic ground plan common to all amniotes to the spe- 

 cialized physiological and metabolic conditions presented by the 

 reptile. 



In detail the reptile is characterized by the relatively enor- 

 mous unfolding of the systemic lymphatic channels and sinuses. 

 Their genesis is identical with that observed in mainmalian and 

 avian embryos, but they obtain a great preponderance in size 

 and extent over the blood-vascular channels. I presume that 

 this is the morphological expression of the lowered combustion- 

 coefficient of the latter in cold-blooded vertebrates. In all three 

 amniote classes the basic principle active in the development 

 of the systemic lymphatic vessels is the formation of independent 

 intercellular mesenchymal spaces, with resulting independent 

 origin of the lymphatic endothelium by direct modification of 

 the mesenchymal cells limiting the early spaces. 



