46 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



complete in case of the lymphatic vessels, as compared with the 

 corresponding vein. The lymphatic system retains, much more 

 perfectly than the venous, in many situations the original embry- 

 onic plexiform type. 



At first the cells limiting the earliest lymphatic spaces are of 

 the usual irregular cuboidal form. As the spaces enlarge, open 

 out and thus become better defined, the limiting mesodermal cells 

 become flattened, and finally assume a typical endothehal char- 

 acter and form. Thus, for example, the endothelial lining of the 

 primitive mesodermal lymphatic spaces {77) is more clearly devel- 

 oped in the 10 mm. embryo, series 120, shown in figs. 8 and 9, 

 than in the corresponding sections of embryo 111, of the same 

 crown-rump measure (figs. 1 to 7). The former embryo is slightly 

 in advance of the latter as regards the development of the parie- 

 tal endothelial lining of the primitive mesodermal intercellular 

 lymphatic spaces. In some instances a few modified mesodermal 

 cells intervene between the cells limiting the lymphatic spaces 

 and the endothelium of the adjacent venous radicle. In others 

 no such intervening cell-layer exists, and the lymphatic space is 

 separated from the venous lumen only by the latter's endothelial 

 wall. In other words, in the extent of the lymphatic anlage, a 

 single-celled membrane furnishes a part of the venous intmia and 

 at the same time contributes to the endothelial definition of the 

 lynii)hatic space. This relation of vein and lymphatic anlage is 

 shown very clearly in fig. 17. The lymphatic space {58), which is 

 closely applied to the medial wall of the left precardinal vein, is 

 only separated from the lumen of the latter by the endothelial 

 membrane which serves to line both spaces for the area of their 

 imitual contact in this stage. Subsequently, with the regression 

 of the left i^recardinal vein, this lymphatic anlage will correspond- 

 ingly enlarge to form an extensive lymphatic plexus, which will 

 eventually toi)()grai)hically replace the vein along which it arose. 

 In order to briefly characterize this relation between vein and 

 lymphatic, McC'lure and I defined in an earlier pubhcation^i these 

 spaces as the "Extra-intimal'' anlages of the systemic lymphatic 

 vessels, with due regard to the relation existing between them and 

 t)ie intimal endothelial lining of the embryonic veins. Themechan- 



