THORACIC DUCT DEVELOPMENT IN THE PIG 405 



were carefully made, but on account of the more restricted accu- 

 racy of this method and the probable errors in the expression of 

 relations and proportions none were used as plate figures. 



The series labelled 'unnumbered' represent incomplete series, 

 embryos of which only the anterior two-thirds or middle regions 

 were sectioned. 



III? REVIEW OF LITERATURE 



It is needless to survey all of the former investigations which 

 have been concerned with the genesis of the lymphatic system ; 

 suffice it to say that the fundamental question in all of them has 

 been: Do the lymphatics arise from the veins, or are they a direct 

 product of the mesenchyme? Only the more recent literature 

 bearing on this query and intimately related to the writer's own 

 observations will be cited and discussed here. 5 



In 1902 Sabin published her first work on the development of 

 the lymphatic system. 6 By the method of injection she found 

 that all lymph vessels, both peripheral and systemic, arise at 

 four centers, two anterior and two posterior, and that they invade 

 the skin as well as the deeper-lying regions of the embryo by a 

 process of centrifugal growth. In other words, consecutive in- 

 jected stages will show the channels springing apparently as a 

 few simple sprouts from these points of radiation and then gradu- 

 ally growing longer and branching in an intricate manner until 

 they have spread throughout the whole body. The two anterior 

 centers, situated one on each side of the neck in the fork of the 

 jugulars, and the posterior ones, inguinal in position, represent the 

 locations of the anterior and posterior lymph hearts, respectively. 

 With the aid of a series of clear diagrams she has mapped out the 

 general course of lymphatic development. Unfortunately, how- 



5 For a comprehensive list of the literature bearing on the development of the 

 lymphatic system, the reader is referred to the following two papers: George S. 

 Huntington : Die Entwickelung des lymphatischen Systems der Vertebraten vom 

 Standpunkte der Phylogenese des Gefass-systems. Anat. Anz., Bd. 39, 1911. 

 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. 



6 Florence R. Sabin: 1902; loc cit. 



