The Origin and Development of the Lymphatic System. 79 



the air cells of an adult lung cannot be reconstructed from sections 

 of uninjected specimens. The degeneration of blood vessels can only be 

 followed by means of complete injections of different stages. 



Kampmeier finds another example of lymph vessels which grow 

 by the extra-intimal replacement of veins in the region where the 

 thoracic duct empties into the jugular sac. In his fig. 8 from an 

 embryo 20 mm. long he shows vessels in which there is a considerable 

 sagging of the endothelium from the surrounding tissue, making the 

 so-called extra-intimal spaces. Kampmeier's embryo measuring 20 

 mm. is about the same stage as mine, measuring 23 mm., since 

 mine was measured before fixation, and hence his figure can be com- 

 pared with fig. 12, in which a lymphatic injection makes it easy to 

 separate veins from lymphatics. As Hoyer says (49b, p. 536) : " Den 

 Verlauf der Lymphgefasse nur an nicht injizierten Serienschnitten zu 

 verfolgen, ist eine miihevolle und zeitraubende Arbeit, deren Ergeb- 

 nisse, wie wir uns selbst iiberzeugt haben, hochstens nur fiir grossere 

 Lymphstamme, deren Lage man kennt, als sicher zu bezeichnen sind. 

 Ist der Verlauf von Gefassen und deren Verastelung noch unbekannt, 

 so lassen sich auf Grund von Serienschnitten nur Vermutungen 

 iiber Verlauf und Verastelung derselben austellen. Erst wenn man 

 fiber Gefasse und ihre Aeste mit Injektionsmasse gefiillt vor sich sieht, 

 erhalt man einen guten Einblick in die Verteilung derselben, deren 

 Verfolgung selbst an nicht injizierten Serienschnitten dann keine 

 wesentlichen Schwierigkeiten mehr bietet." Although it is not pos- 

 sible to be entirely sure in regard to Kampmeier's sections, yet it is 

 at least probable that he is dealing in large part with lymphatics and 

 not with veins at all. The sagging of the endothelium from the 

 surrounding tissue I have found more often with lymphatics than 

 with veins, and yet in tissues fixed in Zeuker's fluid, which Kamp- 

 meier uses exclusively, it may occur in any of the veins and even in 

 the aorta. Kampmeier says, with reference to this group of vessels 

 (fig. 8 and fig. 29 in 66a, p. 460), that they are mesenchymal, peri- 

 vascular spaces into which the jugular sac opens freely; that is, he 

 thinks that the jugular sac in an embryo 20 mm. long opens freely 

 into the tissue spaces. This point is easy to disprove, for in contrast 

 to the early stages of the thoracic duct the jugular sac in this stage 

 is easy to inject. If this were true then every injection of the sac in 

 this stage would show extravasations. This is, however, not so. In 

 fig. 7 is an injection of the sac in an embryo 18 mm. long and I can 



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