THORACIC DUCT DEVELOPMENT IN THE PIG 437 



can readily be distinguished from non-venous, the first by the 

 presence of blood cells and heavier walls, and the second by clear 

 cavities and more delicate walls. 



In series 23a (23 mm. before fixation), an embryo somewhat 

 older than series 194, the discontinuous lymphatic spaces of the 

 postcardinal division are much larger and more conspicuous in 

 the figures. Series 23a is from the Johns Hopkins University 

 Embryological Collection and was injected and prepared by Pro- 

 fessor Sabin. The fixation and preservation of its tissue is excel- 

 lent, and the injection was successfully carried out and is as per- 

 fect as a developing lymphatic channel permits. It was sent to 

 the Princeton Laboratory as a crucial stage in favor of the ' cen- 

 trifugal growth' theory of the origin of the thoracic duct, and 

 therefore the evidence derived from it will seem more significant 

 perhaps than that derived from any other series described. For 

 this reason it will here be dealt with in greater detail. 22 



The right and left limbs of the thoracic duct anlage which are 

 joined to the jugular lymph sac are continuous channels as far 

 back as the points X, X on the drawing, figure 30, and being 

 injected are shown in black on the diagram, figure 13. The fore- 

 most portion of the embryonic duct is in the form of a broad and 

 extensive lymphatic plexus, a typical section of which is repro- 

 duced in figure 14 to illustrate the large size of the channels (3) 

 and the extravasations or leakage {Ex) of the injection mass into 

 the surrounding mesenchyme. To consider the right limb of the 

 duct first, the injected vessel, or vessel confluent with the lymph 

 sac, extends unbrokenly backward and dextrad towards the right 

 postcardinal vein as a slender channel, subjacent to the oblique 

 vessel (7, fig. 30) and at intervals applied to its wall, and termi- 



22 The description of series 23a was presented by the writer before the last or 

 28th session of the American Association of Anatomists (1911) and published in 

 the June number of the Anatomical Record, vol. 6, no. 5, 1912, as a part of a pre- 

 liminary paper, entitled The value of the injection method in the study of lym- 

 phatic development. In this connection it is also well to call attention to Profes- 

 sor McClure's article, A few remarks relative to Mr. Kampmeier's paper on the 

 value of the injection method, etc., which appeared in the same number of the 

 Anatomical Record, and which is a critical analysis of some of the papers published 

 by Professor Sabin and Dr. Eliot Clark on the development of the lymphatic 

 system. 



