The Lymphatic System in Human Embryos. 85 



the vessels turn directly outward and are so cut in cross section. 

 These gaps to be found in serial sections have already been discussed, 

 they occur in thin sections, but much more often in thick ones like 

 these, this embryo being cut at 50 microns, where the slender lym- 

 phatics must often be missed. The extent of thesie superficial lym- 

 phatics has not been shown in the reconstruction, they are readily 

 made out in the skin over the back and hip. There is no difficulty 

 in telling them, they are so sharply lined by endothelium, are empty 

 and about three times the size of the blood capillary. This specimen 

 then shows all the primitive sacsi and their relations to the thoracic 

 duct. It marks also the beginning of the peripheral lymphatic 

 system, both visceral, to the lungs, and superficial to the skin. 



The next specimens consist of a group of four embryos of about 

 the same stage, I^o. 95 measuring 46 mm. and three others (ISTo. 9b, 

 'No. 84 and No. 224) all measuring 50 mm. They all prove to be 

 especially interesting in connection with the development of the 

 posterior lymph sac. In connection with the jugular sac the measure- 

 ments are given in the table. These sacs show certain differences. 

 In No. 95 the transformation into lymph nodes is not extensive and is 

 chiefly at the upper end. No. 224, on the other hand, shows a fine 

 bridging throughout the sac. The other two specimens show an 

 important stage in the evolution of lymph nodes. By referring back 

 to Fig. 14 it will be seen that when the nodes first begin in an embryo, 

 30 mm. long, they consist simply of a thickened connective tissue 

 between a plexus of ducts. But at this stage, 50 mm., there appear 

 round clumps of lymphocytes in the connective tissue bridges. These 

 clumps of lymphocytes are the primary lymph follicles and they 

 occur around the blood vessels of the connective tissue bridges. 

 These primary follicles are well illustrated in Fig. 17, in the femoral 

 lymph node, or in Fig. 18. The evolution of the lymph node depends 

 on the balance between the two elements; the lymph ducts which 

 multiply until they are sinuses and the vascular part with its at- 

 tendant lymphocytes which make the follicles and cords. It will be 

 seen in the figures of these embryos, that in early stages the 

 lymphatic element by far predominates. 



In embryo No. 84, the size of the lymph ducts coming from the 



