The Lyinplirttic System in Human Embryos, Gl 



The first point is well illustrated in the Mall collection; the second, 

 namely the spread of the peripheral lymphatics, needs ample material 

 for injection. It has been worked out only for the lymphatics of 

 the intestine and skin in the pig. 



To return to the first stage, namely that of the origin of the sacs, 

 one can outline the course of development as follows. The jugular 

 lymph sacs begin in an embryo 8 mm. long, the valves are first seen 

 at 10.5 mm. The sac reaches its maximum development at 30 mm. 

 when it attains a size of 5 x 3.6 mm. The beginning of the process 

 of the bridging of the sac, which is the process by which the sac is 

 ultimately turned into a chain of lymph nodes comes early, namely 

 in an embryo 14 mm. long. The process of the transformation of 

 the jugailar, sac into nodes is about complete in an embryo of 80 mm. 

 At 20 mm. there begin to be signs of the formation of the other sacs 

 in the presence of a plexus of veins in the region of the mesenteric 

 sac and the posterior lymph sacs, and at 23 mm. there is a definite 

 retroperitoneal sac and a cisterna chyli. By 24 mm. all three of the 

 sacs are well formed, namely the mesenteric, the cisterna chyli and 

 the posterior lymph sacs. All the sacs together with the thoracic 

 duct are illustrated in Fig. 12, for an embryo of 30 mm., which 

 marks the stage of the completion of the primitive system. The 

 posterior lymph sac which is second in size to the jugular apparently 

 reaches its maximum in an embryo 80 mm. where it measures 

 2.8 x 2 X 3.5 mm. I have no higher stages but judge from its appear- 

 ance that it will soon be entirely cut into lymph nodes. The retro- 

 peritoneal sac, so large in the pig embryos, is always small in the 

 human, and the cisterna chyli is the smallest of all. The thoracic 

 duct is complete in an embryo 30 mna. long. These facts are summed 

 up in the accompanying table. In the table, where one measurement 

 is given, it represents the length of the sac or its antero-posterior 

 diameter; where two measurements are given the first is the length, 

 the second the width or lateral diameter. 



I shall now describe in detail the lymphatics in each of the 22 

 embryos listed in the table. The earliest specimen in the Mall 

 collection to show any traces of the beginning lymphatic system is 

 an embryo (ISTo. 397) 8 mm. long. In this embryo, as shown in 



