52 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



good serial sections of the proper thicknes.s and fixation, and can 

 be reconstructed in the same manner and with the same accuracy 

 and certainty as the blood channels with which they are for the 

 most part so closely associated, although their lumen connects at 

 no point with that of the vascular channels. 



These statements are based, not on isolated observations, but 

 on the close and repeated examination of a very large number of 

 embryos of the same form. It seems curious to me that the pres- 

 ence of the first lymphatic anlages, as above described, should be 

 denied, or, as has been recently done, that the isolated appear- 

 ance of these spaces should be ascribed to the "sudden collapsing" 

 of a lymph vessel. At the time at which they make their first 

 appearance there are no " lymph vessels" to " collapse," no more 

 than there are in the homologous haemal ontogenetic stages blood- 

 vessels in the sense of continuous channels. On the contrary, 

 when they reach their period of most striking development (cat, 

 13-14 mm.) these perivenous lymphatic spaces are, if anything, 

 distended, not only by their fluid contents, but by the remnant 

 of the embryonic atrophying vein which they are in the process 

 of replacing. The only structure showing any sign of " collapse" 

 is the empty endothelial bag of the decadent venule. The spaces 

 become relatively reduced in size in the later stages, after the 

 multiple separate early anlages have fused into a more continu- 

 ous lymphatic channel system. 



These spaces are always present in embryos of the proper stages 

 in the typical position and in constant relation to the venous 

 channels. By following carefully and with sufficient material 

 their further growth and development in succeeding stages, a clear 

 and consecutive picture of systemic lymphatic genesis is given. 



It is noteworthy, in view of the incorrect statements published 

 to the contrary, that these primary anlages of the systemic lym- 

 phatic system develop constantly in embryos of the cat before the 

 definite organization of the jugular lymph sacs. These latter 

 structures, in the 10 mm. cat embryo, are still largely in the con- 

 dition of a perivenous capillary plexus, at a time when the first 

 lymphatic anlages can be distinctly recognized in the axial 

 mesoderm. 



