THORACIC DUCT DEVELOPMENT IN THE PIG 415 



have a course or position subsequently occupied by the thoracic 

 duct does not imply or prove that the latter is a transformation 

 of the former. Embryologists are learning that in order to 

 select and understand all of the steps of embryonic changes, 

 one embryo of each consecutive age will not suffice as was 

 formerly the practice, but a number of embryos of the same 

 stage are requisite. Variations do not begin with the fin- 

 ished organism, but they are potent throughout ontogeny from 

 the beginning onward. Fluctuations of growth either of the 

 whole or of parts are not infrequent, and should we base conclu- 

 sions on a scanty number of embryos the chances are that they will 

 be fragmentary or distorted. There are genetic changes of intrin- 

 sic importance, but so evanescent that we may not even catch a 

 glimpse of them unless we have a series of embryos which ap- 

 proaches the ideal of complete continuity. To produce such a 

 closely graded series may prolong the investigation and make 

 the technic more tedious, but the end result will be more certain 

 and will justify the labor expended. In the following descrip- 

 tions of typical and consecutive stages of the thoracic duct his- 

 tory, and in the discussion of the data presented by them, these 

 general considerations, as well as the various points suggested in 

 the review of earlier investigations, will become more significant. 



The fundamental genetic period of the duct, that is the time 

 between the first appearance of its anlage and the acquisition of 

 continuity throughout its entire extent, is of very short duration. 

 Embryos of 18 or 19 to 23 or 24 mm., depending on individual 

 variations, are the important ones and almost the only ones 

 necessary for this study. In order to demonstrate more clearly, 

 however, the relation which the development of the duct bears to 

 the remainder of embryonic history, we must be aware of the fac- 

 tors and events that lead up to it or, in other words, believe it to 

 be already potential in the period just preceding its actual incep- 

 tion and realization. With this supposition in mind we may 

 artificially divide its history into three phases. 



1 . A veno-lymphatic phase, in which a system of provisional 

 venous channels, or ' veno-lymphatics/ is laid down throughout 

 the entire distance subsequently occupied by the thoracic duct. 



