424 OTTO F. KAMPMEIER 



or any other characteristic which might suggest artifacts due to 

 the action of the preservative are nowhere in evidence. Their 

 endothelium is tense and clear and the enclosed blood corpuscles 

 take a clean and transparent stain. 



During the veno-lymphatic phase the mesenchyme is evenly 

 woven and fairly compact, and lymphatic anlagen, or conspicu- 

 ous and discontinuous spaces, are not present throughout the 

 entire thoracic duct area. Nor can vacuities, fissures, or rents 

 be observed which might be ascribed to unequal fixation, or 

 which differ in any way from the regular intercellular lacunae of 

 the tissue reticulum. 



2. The transition phase {19-22 nun. embryos) 



Having arrived at the second phase of the thoracic duct his- 

 tory, we are confronted with the paramount point at issue, 

 namely, the source and formation of lymphatic anlagen. A 

 critical examination of the stages belonging to this period will 

 disclose three facts of major importance which are impressed 

 upon the observer firmly and constantly. In the first place the 

 longer portion of the thoracic duct anlage arises discontinuously 

 from mesenchymal lymphatic spaces, but it may present various 

 aspects according to its genetic levels or to the degree of relation 

 it bears to immediately surrounding structures, that is, it may be 

 instituted either by extra-intimal spaces, or by spaces in the near 

 environment of the veno-lymphatics but not in contact with them. 

 Secondly, the wall of the entire thoracic duct is a differentiation 

 in situ from the mesenchyme. Thirdly, the development of the 

 thoracic duct proceeds in a general antero-posterior direction; 

 for example, in series 194, a 20 mm. embryo, lymphatic develop- 

 ment has made considerable progress in the anterior or precardinal 

 division, has just been initiated in the middle or postcardinal divi- 

 sion, and is totally lacking in the supracardinal division. 



(A) Precardinal division. In the collection of pig embryos 

 studied by the writer the first instances of incipient lymphatic 

 anlagen are found in series 168 (19 mm.) along a limited extent of 

 the precardinal veno-lymphatics as far back as the anterior half 

 of the oblique vessel (fig. 28) . Located in the path of the poten- 



