THORACIC DUCT DEVELOPMENT IN THE PIG 451 



pre- and postcardinal divisions. Almost concurrently the changes 

 producing the cisterna chyli and its connection with the posterior 

 and mesenteric lymph sacs have been active, so that the segment 

 of the duct anlage in the territory of the periaortic vessels, at the 

 level of the mesonephroi, is the last one to acquire continuity 

 among the lymphatic spaces and to make the thoracic duct an 

 unbroken tube from one extremity to the other. The reason 

 that those portions of the duct nearest to the lymph sacs are 

 developed first, that the vacuolation of the mesenchyme, the 

 formation of isolated spaces, and their confluence proceeds in 

 a general centrifugal direction is not far to seek, being probably 

 inherent in the explanation which would make the accelerating 

 pressure of the lymph stream towards its points of entry to the 

 veins sufficient to account for, or at least sufficient to furnish 

 the stimulus for, the progressive occurrence of such phenomena. 

 The elongation of lymphatic spaces and their fusion finally 

 into a continuous channel, as well as the growth of their cavities 

 in diameter, is accomplished by the same process which gave 

 origin to them, namely, by the disintegration of tissue fibrils and 

 the concentric addition of spaces. Figure 22 represents a typical 

 transverse section from the precardinal division of an early lym- 

 phatic stage and illustrates very plainly how the increment in 

 size of the thoracic duct anlage is effected. An accurate camera 

 lucida drawing, figure 23, of a portion of the same section is also 

 inserted here to bring out more distinctly some of the details 

 which may be obscure in the microphotograph due to the differ- 

 ences in focus. Both of these figures demonstrate that the duct 

 (5d) at this embryonic period enlarges by a process of growth not 

 from within outward but from without inward, by the admission 

 of ad j acent mesenchymal spaces to its channel. The strands inter- 

 secting the lumen are therefore indicative of successive lines of 

 fusion or a measure of its gradual growth. From the facts just 

 stated and the exceedingly indefinite boundaries of the anlage, 

 we should expect the absence at this time of any kind of demarca- 

 tion membrane between the cavity and the interstices or lacunae 

 of the surrounding tissue. That such is really the case is borne 

 out by experiment. Both the veins and the thoracic duct of an 



