THORACIC DUCT DEVELOPMENT IN THE PIG 433 



pact in texture than the tissue surrounding it is evident at a 

 glance. Within it the cells are fewer in number, and the tissue 

 fibrils, which appear to be more delicate than those of the mesen- 

 chymal reticulum elsewhere, enclose larger interstitial openings or 

 tissue spaces. In longitudinal extent this potential duct-anlage, 

 as we may call it, occurs along a considerable portion of the pos- 

 terior half of the oblique vessel but varies from section to section 

 in its definition. Often, distinct vacuoles appear suddenly in 

 it, continue through several sections, and as suddently disappear. 

 Only these 'centers of space-formation,' however, can be repro- 

 duced in a model (fig. 29), the remainder of the anlage being as 

 yet too indefinite to warrant reconstruction. 



The atrophy of the oblique vessel in this specimen, series 194, 

 serves also as a typical example of the atrophy of all temporary 

 or redundant venous pathways, both of the veins which are the 

 immediate antecedents of lymphatics and of those veins which are 

 not so intimately associated with the development of a lymphatic 

 channel. In the embryo from which figure 28 was drawn the 

 oblique vessel (7) is still complete and continuous with the main 

 venous trunks; in figure 29, on the other hand, it is seen to be 

 broken up into irregular segments, some of which are replaced by 

 extra-intimal spaces, and others gradually diminish in size and 

 disappear in the mesenchyme adjacent to an incipient lymphatic 

 anlage but with an appreciable amount of tissue between them. 

 Thus it is evident that the vanishing segments of redundant 

 venules and the growing segments of potential lymphatics may 

 exist side by side in the same section. But now it may be asked, 

 what distinguishes the one from the other, what basis is there for 

 naming this one a lymphatic rudiment and that one a venous 

 remnant, and how can both be followed to their ultimate fates 

 without confusion? The distinction between these two vascular 

 structures can easily be recognized beyond the possibility of a 

 doubt. A lymphatic segment, here specifically a thoracic duct 

 anlage, is invariably characterized by a very clear lumen and, if 

 it is in the formative stage, by the absence of a clear-cut and 

 specialized lining, as shown in the microphotographs already 

 mentioned (figs. 6, 7, 8 and 10) ; whereas, the segments of a ven- 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO 



