146 ADAM M. MILLER 



the tissue spaces move to and fro in response to the heart-beat, 

 as has been clearly observed in the living chick blastoderm, the 

 assumption is justifiable that their movement, with the resultant 

 friction and pressure upon the adjacent protoplasmic elements of 

 mesenchymal tissue, assists in the flattening of these elements 

 and the consequent formation of endothelial cells. 



The hydrodynamic factors of pressure and friction of the inter- 

 stitial fluid substance upon the protoplasmic elements of the 

 mesenchyme, which were first discussed by Thoma (19) in con- 

 nection with blood vessel formation and have already been noted 

 in this particular case of development of lymphatics, together with 

 the additional factor of pressure and friction of oscillating blood 

 cells would, in the opinion of the writer, afford adequate mechanical 

 means of changing the irregular plastic elements of the mesen- 

 chymal syncytium into the endothelial cells of the vessel wall. 



IV. MORPHOGENESIS 



Up to this point we have been considering the histogenetic 

 changes which occur in the mesenchj^mal tissue, resulting in the 

 formation of lymph channels and their endothelial lining and of 

 blood cells. It has been demonstrated that aggregations of the 

 developing blood cells, identical with the mesenchymal 'cords' 

 described by Sala, appear along the lines of the future thoracic 

 duct. It has been shown also that the rudiments of the thoracic 

 duct develop as isolated spaces and channels in the mesenchyme, 

 that the endothelial lining of the channels is derived directl}^ 

 from the mesenchymal cells forming their borders and that the 

 channels coalesce to form a plexus. It is our object, under the 

 head of morphogenesis, to trace the subsequent history of the 

 isolated channels and the plexus formed therefrom. 



Chick embryo of six days and sixteen hours, 13.5 mm. {Columbia 

 Embryological Collection, series no. 4^6). Reconstruction, ventral 

 view. Figure 20. The incipient stages of thoracic duct develop- 

 ment are shown about this time. The lymph spaces {17) are three 

 in number, two on the right side and one on the left, situated 

 in the mesenchyme ventro-lateral to the aorta {I) about midway 

 between the exits of the celiac (-5) and superior mesenteric arte- 



