The Origin and Development of the Lymphatic System. 69 



iows plainly in fig. 7 from an embryo much smaller, namely, one 

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

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

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

 lit be found, in sections. It is probable that the lymphocytes begin 

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

 jvelop almost simultaneously in the primary and in the secondary 

 lands. 



The plexus stage of developing lymph glands" has been shown by 

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

 y. 10 (Sabin 131). 



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

 immed up in a word by a quotation from Eanvier (p. 1038 in 117) : 

 Un ganglion lymphatique est un angiome caverneux lymphatique qui 

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

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

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

 es in the trabeculas. A comparison of Jolly's (59) plates from lymph 

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

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

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

 lt glands. 



The formation of lymph sinuses can be best understood by beginning 

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

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

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

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

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

 mammals. On this account the complete covering of endothelium 

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

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

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

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

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

 aus can be seen in a single section. In the lower part of the specimen 

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

 bile in the upper part the septa are in places already reduced to a 

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



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