438 ELEANOR LINTON CLARK 



The posterior lymph sac (lymph heart) and the supra-scapular 

 sac are both formed by the flowing together of neighboring main 

 channels whose walls remain for a time as thin trabeculae. And 

 over the posterior pelvic region a sac is formed sometimes by the 

 expansion of a portion of a single main channel, or, more fre- 

 quently, by the enlargement of two or more nearby vessels. 



The observations of the character of the l3niiph-flow and the 

 pressure conditions in various regions and at different stages 

 have helped to explain the development of these embryonic 

 lymph sacs, since they have shown that a lymph sac forms at 

 any point in the early lymphatic system where conflicting pres- 

 sures occur. For example, a sac develops in a place such as the 

 supra-scapular region where there is an obstacle to the outlet 

 of fluid, in regions like the posterior lymph heart and the deep 

 jugular plexus where the pressure in the lymphatics encounters 

 the side pressure in the connecting veins, or at a point of hesi- 

 tation such as the superficial pelvic region, where the lymph-flow 

 is now in one direction and now in the opposite. 



In describmg the development of the lymphatics of the chick, 

 there appears to be no adequate reason for a division into pri- 

 mary lymph sacs and peripheral lymphatics, since the mode of 

 development is essentially the same throughout the early lym- 

 phatic system after its differentiation . The peripheral lymphatics 

 have spread over the surface of the body before any lymph sac 

 develops and there appears to be no real distinction, either in 

 the time of their appearance or in the manner of formation, of 

 the sacs which may develop near the venous connections of the 

 early lymphatics (in those regions where the first lymphatic 

 capillaries make their appearance in much younger embryos) 

 and the sacs located in other portions of the lymphatic system. 

 Sacs may form at any point in the early system where the mechan- 

 ical conditions favor such an enlargement and all of the sacs are 

 secondary structures. The primary form of the developing 

 lymphatic system, in chick embryos, is a rapidly growing, richly 

 anastomosing, irregular blood-filled plexus out of which channels 

 or ducts and lakes or sacs develop secondarily, in accordance 

 with the circulation of lymph which is determined by pressure 



