THORACIC DUCT DEVELOPMENT IN THE PIG 409 



However, she firmly believes that the 'lymphatic anlagen' of 

 Lewis appear isolated only in the study of serial sections, and that 

 their continuity can be demonstrated by the method of injection. 

 In other words, "in complete injections there are no vessels 

 which have not received the injecting mass," but "in partial 

 injections and uninjected specimens there are endothelial-lined 

 vessels" which appear to be broken up into segments, so that con- 

 tinuity "can be traced only with difficulty or not at all." In 

 this same article she admits the presence of true mesenchymal 

 spaces "which undoubtedly contain lymph," but tacitly assumes 

 that they "are to be excluded from the lymphatic system morpho- 

 logically." They are isolated and cannot be injected and they 

 do not possess a clearly defined intima. 



A year later, in 1909, Sabin published her observations on the 

 development of the lymphatic system in human embryos. 14 

 In this work she reaches and emphasizes essentially the same 

 points as in her previous investigations. In the case of the. 

 thoracic duct, however, she hesitates to take a definite position. 

 She believes that it originates as outgrowths of the jugular lymph 

 sac and cisterna chyli, but she states in this connection that the 

 "thoracic duct has proved to be the most difficult part of the 

 lymphatic system to work out for this reason, we have not yet 

 found a way to inject it in early stages and uninjected sections 

 are not adequate." Further: 



The question is, does the duct develop from multiple anlagen from 

 the azygos veins for which there is no proof except that lymphatic ves- 

 sels can be seen in sections adjacent to these veins, or does' the duct 

 grow from the two sacs, the cisterna chyli and the jugular one. For 

 the second view the evidence is also weak, it consists in this, that other 

 lymph ducts wherever we can study them grow from the sacs; and sec- 

 ondly in pig embryos and in human embryos one can trace a duct for- 

 ward from the cisterna chyli and caudal ward from the jugular sac, and 

 in later stages these two ducts have joined. The weakness of this evi- 

 dence lies in the fact that in earlier stages the picture is always liable to 

 be confused by Lewis' multiple anlagen. 



14 Florence R. Sabin: On the development of the lymphatic system in human 

 embryos, with a consideration of the morphology of the system as a whole. Am. 

 Jour. Anat., vol. 9, 1909, pp. 43-90. 



