The Origin and Development of the Lymphatic System. 81 



of such spaces are the constantly occurring spaces in the parietal 

 pleura on either side of the vertebra. They are always present in the 

 stages in which the thoracic duct is developing; they are adjacent to 

 the vascular zone just internal to the ribs and they never have any 

 relation to lymphatics. A definite, constant group of such spaces is 

 also to be found in the subcutaneous tissue of the mid-dorsal line of 

 certain stages and they never become lymphatics. The pia-arachnoid 

 and perineural spaces likewise never become lymphatics. In fact, 

 large as well as small groups of such spaces are a constant occurrence 

 in sections of embryos ; sometimes near lymphatics and sometimes far 



Fig. 19. — Diagram to show some of the various structures which Hunt- 

 ington and McClure have described as lymphatics, a. = true lymphatic 

 capillaries; b. = Mayer-Lewis anlagen, also true lymphatic capillaries; c. 

 — extra-intimal hypothetical lymphatic capillaries; d. = tissue spaces. 

 The perineural spaces are not shown. The veins are striped, lymphatic 

 capillaries dotted, and hypothetical lymphatic capillaries are cross-hatched. 



from them. The selection of certain tissue spaces as destined to 

 become lymphatics is an arbitrary selection and brings us back to the 

 confused standpoint of the earlier embryologists ; a standpoint which 

 would indeed justify the view of B artels that the question of the 

 relation of the lymphatics to the tissue spaces is " eine philosophische, 

 keine anatomische Frage." 



The difficulty of finding out the method of growth by observing 

 sections is best illustrated by following the different theories of their 

 growth in the pages of Huntington and McClure. Some of these 

 methods are shown in the diagram, fig. 19. This development of the 



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