208 FREDERICK TILNEY 



communication with the thoracic duct approach of the lymph sac. 

 On the left side, the two divisions are also present, but neither 

 has made connection with other parts of the lymphatic system. 



Lymphatic organization in the 23 mm. embryo 



Axial lymphatic organization has been carried to its consum- 

 mation in the 23 mm. embryo. The imperfectly crystallized 

 conditions of the next younger specimen have already marked 

 out the line along which this development would proceed. 



Body of the lymph sac (fig. 10). The body of the sac on both 

 sides lies in front of the 3rd, 4th, and 5th cervical ganglia, the 

 6th ganglion and the interspace above it lying entirely below 

 the caudal limit of the vesicle. The long body axis now bears 

 the ratio of 17 to 1 to the long axis of the lymph sac, whereas 

 in the 20 mm. embryo this ratio was 10 to 1. Measurements 

 of the sac as computed from the mounted sections show that 

 this decrease in size is not merely relative but absolute, as the 

 following values of the long axes show: 



The ventro-dorsal and transverse diameters also show a decrease- 

 That this attenuation has chiefly affected the more ventral por- 

 tion of the sac is shown by the fact that the two nerves (4th 

 and 5th cervicals) which in the 20mm. embryo passed through 

 the sac have now freed themselves, while it also appears that 

 the process which released them has at the same time produced 

 a neck in the sac itself. By this neck the sac approaches the 

 jugular vein and attains its ultimate systenal lymphatic and 

 venous connections. It will be obvious, therefore, that this cervi- 

 cal portion corresponds in general to the caudal extremity in 

 the sac of the next younger specimen. 



Processes of the sac. Several of the cephalic processes maybe 

 traced for a considerable distance into the head region and un- 



