DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 279 



The foregoing offers a very concise and much abbreviated 

 history of the jugular lymphsacs as viewed from the standpoint 

 of the more recent investigations. 



McClure and I, pubhshing our detailed results in 1910, started 

 our investigation of the genesis of the sac about at the stage 

 shown in figure 7 of the above series of schemata. 



In spite of the fact that we noted, described and figured the 

 detached and isolated blood-islands and the independent inter- 

 cellular mesenchymal spaces of the earlier stages, the whole pic- 

 ture, in the abseilce of the correlated evidence since then obtained 

 by the study of other amniote embryos, and without the recently 

 introduced special blood stains, forced us to adopt the view that 

 the anlages of the jugular lymph sacs were primarily venous 

 derivatives from the pre- and postcardinal veins. We would 

 today define them as prirnanj lymphatic channels, developed, as 

 are all true lymphatic vessels, independently of the veins, but 

 charged during a definite embryonal phase with the physiologi- 

 cal duty of conveying the products of mesenchymal haemopoesis 

 to the circulating stream of the veins. They thus appear, from 

 their inception, as components of the future lymphatic system, 

 sharing the same type of independent genesis with all the re- 

 maining lymphatic channels, but temporarily haemophoric in 

 their physiological relation to the vascular system as a whole. 



I have in a number of previous publications formulated a 

 genetic interpretation of the development of the mammalian 

 lymphatic system which, in the light of the more recent investi- 

 gations, should be modified in one respect. Instead of regard- 

 ing the jugular lymphsacs, the connecting links between the 

 independently developed systemic lymphatics and the veins, as 

 being primarily' venous in their derivation, it would be, in our 

 present knowledge, more correct to define them as primitive 

 lymphatic channels whose early haemoplioric function serves to 

 differentiate them sharply as a group from the remaining sys- 

 temic lymphatics, developed at a later period through the same 

 genetic confluence of intercellular mesenchjnnal spaces, but with- 

 out direct relation to the veins or to the blood-contents of the 

 haemal system. It is here in all probability that the clue is to 



