THORACIC DUCT DEVELOPMENT IN THE PIG 459 



trates last;" 24 and concerning all 'mesenchymal spaces/ Clark 

 says: "They occur most often around bloodvessels, and are almost 

 certainly to be interpreted as shrinkage spaces, or spaces caused 

 by the retraction of the mesenchyme processes made possible by 

 slight rents produced in the preparation of the sections.' 25 That 

 these objections are wholly without foundation, at least in the 

 case of the developing thoracic duct in the pig, is conclusively 

 shown by the following observations : In the first place, the mesen- 

 chymal spaces termed lymphatic anlagen spring into existence at a 

 definite period of embryonic history and invariably in a definite 

 position. The embryos grouped under the veno-lymphatic phase 

 are without an exception the younger embryos, and, although they 

 were preserved in the same fixatives and the preparation of the 

 sections followed the same methods as those employed for the 

 specimens of the second phase, they do not show or even suggest 

 instances of such anlagen. Secondly, in all of the earlier stages 

 of the transition phase these lymphatic spaces can only be ob- 

 served in the precardinal division, namely, in the territory of the 

 most anterior segment of the thoracic duct which is formed first ; 

 in the postcardinal and supracardinal divisions the mesenchyme 

 is still uniform, and the veno-lymphatics are functional and 

 joined to the parent veins. In the later stages of this phase the 

 last two segments repeat the history of the first. Thirdly, all 

 of the extra-intimal spaces in the thoracic region of the writer's 

 specimens occur only in connection with those venules which 

 have been severed from their venous trunks and which lie topo- 

 graphically in the pathway of the potential duct. As far as the 

 writer was able to determine, other abandoned veins existing 

 in the same general areas and in the 'center of the body/ but not 

 antecedent in position to the duct or to any of its tributaries, 

 never manifest extra-intimal figures in their atrophy. All of 

 the functional veins possess a normal and distended endothelium. 



24 Florence R. Sabin: A critical study of the evidence presented in several recent 

 articles on the development of the lymphatic system. Anat. Rec, vol. 5, no. 9, 

 1911. 



25 Eliot R. Clark: An examination of the methods used in the study of the devel- 

 opment of the lymphatic system. Anat. Rec, vol. 5, no. S, 1911. 



