GO Florence R. Sabin. 



These spaces along the nerves, which may be termed perineural 

 spaces, are especially important to note both on account of their 

 physiological significance and because they have been confused with 

 lymphatics. They may be injected from the space around the spinal 

 cord and they are especially large around the growing tips of the 

 nerves. Their constancy, their presence in perfectly prepared speci- 

 mens, and especially their size at the growing tips of the nerves, leads 

 one to think that they are physiologically of great importance to the 

 nerves, but they never form a part of the lymphatic system. ^N"© 

 injections of these spaces ever run over into the lymphatic system. 

 An injection into the developing arachnoid spaces around the spinal 

 cord will often pass into the veins, entering them around the fourth 

 ventricle, but I have never succeeded in injecting any lymphatic 

 vessels from the arachnoid nor in tracing any lymphatic vessels to 

 the arachnoid, so that I believe the older anatomists, for example 

 Breschet, were right in believing that the lymphatic system does not 

 drain the great arachnoid lymph space which rather retains its 

 primitive relation to the veins while other parts of the body become 

 drained by a new system of capillaries, namely the lymphatics, 

 derived from the veins. 



In studying Professor Mall's collection it seems that there are two 

 stages to be made out in the development of the system as a whole. 

 This ha^ been illustrated in the table on the next page. The first 

 includes a study of the origin of all of the primitive sacs and their 

 fusion into a primitive lymphatic system through two factors, namely 

 the formation of the valves of tlie jugular sacs which make the per- 

 manent openings into the veins, and secondly the connection of the 

 various sacs by means of the cisterna chyli and thoracic duct. This 

 period includes embryos up to 30 mm. in length, of which there 

 are seventeen in the series. This first stage may be divided into 

 two periods, one of which there are fourteen specimens, measuring up 

 to 20 mm., which have the jugular sacs alone ; the other shown in 

 three specimens, which mark the time of origin of the other sacs 

 and of the thoracic duct. 



The second stage involves the transformation of the sacs into the 

 primary lymph nodes and the spread of the peripheral lymphatics. 



