148 ADAM M. MILLER 



lateral to the aorta (i) near the exit of the celiac artery (5). 

 Other spaces and channels of a similar nature are seen along the 

 dorsal aortic root (2), thus continuing the same general line of 

 lymphatics toward the jugular lymph sac (15) with its thoracic 

 duct 'approach' {15a). Some of these early lymphatics are of 

 considerable length and have acquired a distinct endothelial 

 lining. Others are simply enlarged spaces in the mesenchyme 

 exactly like those described in the preceding stage (figs. 13 and 14). 

 There is not yet any plexiform arrangement of the channels. 



The principle underlying the formation of the longer channels 

 is in the main the enlargement and coalescence of two or more of 

 the original smaller spaces or channels. For instance, two isolated 

 spaces or channels lying near each other in the mesenchyme 

 increase in size until eventually they flow together to form a 

 single space or channel. Or, the same process occurs in several 

 spaces or channels in a discontinuous series until all the members 

 of the series flow together and thus form a continuous channel. 



There is also in all probability some proliferation of the endo- 

 thelium lining the channels. In fact there is no valid reason for 

 not assuming that, after the inception of a given vascular channel, 

 either hemal or lymphatic, along with its increase in size there 

 is a concomitant proliferation of its endothelial cells. The crucial 

 point, however, is the origin of the channels. The evidence at 

 hand points clearly to the origin of the lymphatics constituting 

 the rudiments of the thoracic duct directly from the mesenchymal 

 interstitial spaces in the manner previously described, the endo- 

 thelium of the lymphatics representing mesenchymal cells which 

 are modified in accordance with the new conditions of pressure 

 and friction. 



In this embryo (series no. 465) the rudiments of the thoracic 

 duct (fig. 21, 17) are isolated. There is no connection between 

 these and the hemal vascular system; nor on the other hand is 

 there any communication with the jugular lymph sacs (15). The 

 thoracic duct 'approach' of the lymph sac, which the duct even- 

 tually joins, is well developed at this stage (15a) but a considerable 

 distance intervenes between the 'approach' and the rudimentary 

 duct. 



