The Origin and Development of the Lymphatic System. 75 



taelied blood vessels, and hence the hypothetical Mayer-Lewis anlagen 

 do not exist, that is to say, they are not anlagen of lymphatics, 

 but are parts of a continuous vessel. It is clear that reconstructions of 

 growing lymphatics are valuable, even though they show the lym- 

 phatics as rows of beads, provided only the true lymphatics are in- 

 cluded; that is to say, the endothelial-lined vessels. For example, 

 Lewis (76, figs. 7 and 8) shows that the early lymphatics in the rabbit 

 grow along the thoraco-epigastric vein. 



2. EXTRA-IXTIMAL AND PERINEURAL SPACES AND FENESTRATION. 



Huntington and McClure (51) began their work on the lymphatic 

 system in 1907 with a study of the jugular lymphatics in the cat. 

 They thought that lymphatics began as clefts between the intima, 

 the vein and the surrounding mesenchyme, so that for a time the 

 wall of a lymphatic was half venous endothelium and half mesen- 

 chyme. These spaces they called " extra-intimal " lymphatics. As 

 far as the jugular lymphatic sacs were concerned they subsequently 

 abandoned this idea, accepting the theory that the jugular sacs are 

 derived from the veins (Huntington and McClure 54), but they have 

 since revived the extra-intimal theory in connection with the thoracic 

 duct (Huntington 58). 



Their work on the jugular sacs has been given in section IV. In 

 general they accept the theory of the venous origin of the jugular 

 sacs, but they confuse the picture (1) by the inclusion of some peri- 

 neural spaces (fig. 22, Huntington and McClure 54) and (2) by the 

 idea of fenestration.. 



The perineural spaces are an interesting phenomenon. I con- 

 sidered them in my first paper in. 1902. Along the entire central 

 nervous system and following the peripheral nerves are dilated tissue 

 spaces. Within the spinal canal these spaces are the anlage of the 

 spaces of the pia-arachnoid. Along the nerves we call them perineural 

 spaces. All sections of embryonic tissue show them. They are of 

 undoubted significance for the physiology of the growing nerves, but 

 they never bear any relation to the lymphatics. Injections of them 

 outline the nerves and never enter lymphatic vessels— occasionally 

 they can be injected from the pia-arachnoid, but usually the dense 

 tissue between the vertebrae prevents this. 



The question of fenestration was considered in section IV. Paral- 

 lel veins are formed by the same process of sprouting of endothelium 



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