538 



owing to the changed relations brought about by an elongation of this 

 region, and, in the writer's experience, has only been made possible 

 by a study of wax reconstructions. The determination of the aulages 

 of that portion of the ducts which arises along the ventral surface of 

 the azygos veins, however, is an extremely easy matter and, owing to 

 the simpler relations which exist in the region of the body traversed 

 by these veins, we find here what 1 regard as an indisputable de- 

 monstration of the thoracic and right lymphatic ducts in the process 

 of being spHt off from the veins. 



A transverse section taken through the thoracic region of a 16 mm 

 cat embryo (Fig. 7) ^) shows the position occupied by the thoracic and 



Right azygos 



Left iizvgos 



Right lymph, duct Aorta 



Fig. 7. Microphotograph of a transverse section through the thoracic region of a 

 16 mm cat embryo showing the thoracic and right lymphatic ducts and their relations 

 to the aorta and the azygos veins, 



right lymphatic ducts with respect to the aorta and the azygos veins. 

 Although the two ducts lie in close contact with the azygos veins they 

 do not, in this nor in older embryos, communicate with them at any 

 point along their course. On comparing Fig. 7 with Figs. 8 and 9, 

 which are microphotographs of two successive sections through the 



1) The sections represented by Figs. 7, 8 and 9 were photographed 

 from the anterior surface so that the left azygos vein appears on the 

 right side of the figure. 



I take this opportunity of thanking my Assistant, Mr. C. F. Silvester, 

 for his kindness in preparing the photographs for this publication. 



