68 Florence B. Sabin. 



by endothelium is best brought out by total mounts of silver nitrate in 

 jections. This has been done for the retroperitoneal sac in the pig 

 When the primary sacs are thus completely bridged by these bandi 

 they are practically a dense plexus of lymphatic capillaries and art 

 therefore in the first stage of the development of lymph glands. A 

 this stage the connective tissue septa are undifferentiated and contain 

 only mesenchyme and blood capillaries. 



We are now in a position to consider the question of which are th 

 first lymph nodes in the embryo. This point has been discussed b; 

 Lewis (77) and Sabin (131 and 134). It depends on the criterioi 

 used. If the criterion is recognizing the formation of a plexus o 

 lymphatic capillaries with undifferentiated connective tissue bridge 

 as the anlagen of a gland, then it is clear that the primary lymp] 

 glands, that is, those that come from the lymph sacs, are the firs 

 lymph glands to begin for each region of the body. Thus the jugular 

 subclavian sac in the human embryo is extensively bridged at 30 mm 

 and is therefore a plexus of lymphatics, the anlage of a lymph gland 

 At this stage we are agreed there are no lymphocytes (Kling (72) 

 Lewis (77) and Sabin (131)). If, on the other hand, the criterioi 

 is the development of the first lymphocytes in the body, we mus 

 say that this point has not been reinvestigated since all the primar 

 lymph nodes of the embryo have been determined. Saxer (13£ 

 p. 381) notes that lymph nodes are beginning in the neck, the pos 

 terior mediastinum, the retroperitoneal tissue, the outside of bot 

 hips and along the Wolffian bodies in bovine embryos 4.5 cm. lon£ 

 and in sheep embryos of the same length describes undoubted wandei 

 ing cells in the same areas. Kling (72) has no specimens of huma 

 embryo between 31 mm., where, he says, there are no lymphocytes i 

 the axillary glands, and 70 m., when lymphocytes are present in man 

 glands. Lewis (77) finds lymphocytes in human embryos 42 mn 

 long. He finds that they occur practically simultaneously in tb 

 glands around the internal jugular vein (primary lymph nodes 

 and certain " isolated subcutaneous lymph glands," of which he figur< 

 one or two along the linguo-facial vein and its branches. The relatio 

 of these glands to lymphatic vessels will be readily made out by 

 comparison of fig. 8 and fig. 16. Figure 8 shows the ducts from tl 

 sac along the linguo-facial vein at a stage even younger than Lewis 

 isolated glands, namely, a pig measuring 3.5 cm. Moreover the b 

 ginning cervical plexus from which these submaxillary vessels con 



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