162 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



jugular lymph sac appear side by side, dorsal to the left common 

 jugular vein, between the thyro-cervical artery and the sympa- 

 thetic nerve. 



5. Section 12 of the same slide (fig. 61) is caudal to the blind ter- 

 minal of the thoracic duct approach. A radicle of the dorso-med- 

 ial venous plexus (16) lies close to the dorsal circumference of the 

 common jugular vein (26) between the thyro-cervical artery (24) 

 and the sympathetic nerve (1), approximately in the situation 

 occupied in the sections further cephalad by the terminal of the 

 thoracic duct approach. 



6. In the following section (fig. 62) this venous tributary opens 

 into the main vein. 



A comparison of the structures labelled respectively 16 and 12 

 in figs. 61 and 59 will show their close topographical correspondence 

 and will suffice to indicate how easily the latter, a valid portion of 

 the future thoracic duct, might be confused with the former, a 

 dorso-medial venous terminal. Hence, in comparing successive 

 stages, the greatest care is necessary in order to avoid the mistake 

 of considering a lymphatic channel of the later period to be the 

 direct derivative of a ' venous outgrowth' as seen in an earlier em- 

 bryo. Other embryos of this stage will give equally conclusive 

 pictures of the juxtaposition of thoracic duct approach (12) and 

 dorso-medial venous tributary (16) in their various mutual rela- 

 tions (figs. 41, 42, 49, 50, 55). 



Of course, caudal to the level at which in this stage the thoracic 

 duct approach ends blindly, and before the independent lymphatic 

 channels, which are subsequently (15 mm. and 15.5 mm.) to join 

 it, are developed, the sections will only show the dorso-medial 

 venous branches entering the main channel 



But these venous terminals of a well defined plexus are in no 

 sense ' outgrowths' from the veins, nor should they be considered 

 as anlages of a lymphatic structure which does not appear in the 

 particular section figured, because it has not as yet developed at 

 that particular level, although, as in the case in point, it may be 

 perfectly evident and coexisting with the 'venous outgrowths' 

 few sections further cephalad in the same embryo. 



The venous plexuses, both in the prevertebral and in the ventral 



