Mesenteric Sac and Thoracic Duet in Enibrvo Piff 



315 



dnct and the cervical lymphatics, so that when the embr3'o reaches 30 

 mm. the three integral parts of the lymphatic system have all become 

 united to form a completed duct. The connection between the duct and 

 the cervical lymphatics is established as follows : In an embryo between 

 25 and 26 mm. the duct, which is at first bilateral, extends up to a 

 point about opposite the arch of the aorta, where the two main trunks 

 become fused. The cervical lymphatics, by this time, liave orown down 





Fig. 8. — Transverse section through the beginnings of the mesonephric arter- 

 ies in an embryo pig, 23 mm. long. The figure shows the definite mesenteric 

 sac in the region between the mesonephric arteries which was occupied by 

 the venous plexus in the earlier stages. A., aorta ; M. 8., mesenteric sac, 

 showing the upgrowth of capillaries around the aorta ; S. C. V., sub-cardinal 

 veins ; Wl B., Wolffian body ; G. A., genital anlage ; M. A., mesonephric artery. 



to this same level and connection between the two is established by small 

 anastomosing channels, scarcely larger than capillaries. Gradually, as 

 development takes place, these become distended, until a duct of uniform 

 calibre is formed when the embryo reaches a stage, above 33 mm., from 

 which time complete injections of the lymphatic system can be made 

 without difficulty. Such an injection is shown in Fig. 10, made from 

 a cleared specimen 5.5 cm. long. 



This specimen gives a good idea of the primitive lymphatic system. 

 The cervical h^mph sac is shown now turning into lymph nodes. The 



