The Origin and Development of the Lymphatic System. 69 



shows plainly in fig. 7 from an embryo much smaller, namely, i ne 

 measuring 18 mm., and they are readily injected in embryos 30 mm. 

 long, fig. 2 (Sabin 130). Thus it is almost certain that the ducts for 

 Lewis's early facial glands in human embryos were present, but could 

 not be found, in sections. It is probable that the lymphocytes begin 

 in human embryos which measure about 40 mm. and they probably 

 develop almost simultaneously in the primary and in the secondary 

 glands. 



The plexus stage of developing lymph glands has been shown by 

 reconstruction by Kling (72) and Lewis (77), and by injection in 

 fig. 10 (Sabin 131). 



The whole question of the structure of a lymph gland may be 

 summed up in a word by a quotation from Ranvier (p. 1038 in 117) : 

 •* Un ganglion lymphatique est un angiome caverneux lymphatique qui 

 a etc d'abord angiome simple." The development of a gland involves, 

 however, two processes: (1) the formation of lymph sinuses out of 

 lymph plexuses, and ( 2 ) the formation of lymph cords and lymph folli- 

 cles in the trabecular. A comparison of Jolly's (59) plates from lymph 

 glands in birds, Kling's (72) figures of human lymph glands, and 

 my own (131) from the developing glands in the pig. will show that 

 the relative proportion of these two parts varies exceedingly in differ- 

 enl glands. 



The formation of lymph sinuses can be best understood by beginning 

 with Jolly's figures. He shows that in the birds, instead of the mam- 

 malian sinus, there is a diffuse plexus of lymphatic vessels with lym- 

 phocytes in the septa between the vessels. He also brings out the 

 Eai t that very large vessels may pass directly through the gland. Thus 

 it is clear that the sinus of birds is a simpler structure than that 

 of mammals. On this account the complete covering of endothelium 

 is very plain in Jolly's figures. Out of a plexus of capillaries such as 

 Jolly shows, the mamalian lymph sinus is formed, as can be seen 

 in fig. 15 (Sabin 131 ), by an increase in the density of the lymphatic 

 plexus until the septa are reduced to a framework of reticulum covered 

 by endothelium. In this figure all the stages of the formation of a 

 sinus can he seen in a single section. In the lower part of the specimen 

 every lymphatic vessel is plain with its complete lining of endothelium, 

 while in the upper part the septa arc in places already reduced to a 

 line. This stage is from an embryo pig 24.5 cm. long, which is the 



