THORACIC DUCT IN THE CHICK 141 



be considered in the subsequent description of the formation of 

 the lymphatic spaces and channels. After the seventh day, as 

 the lymphatic channels in this region develop and establish com- 

 munication with the jugular lymph sacs, the intravascular masses 

 of blood cells decrease and, by about the eleventh da}^ are reduced 

 to a few groups scattered through the lymphatic plexus (cf. figs. 

 24, 27 and 28). A considerable number of extra vascular groups 

 persist, however, until the fourteenth or fifteenth day, or even 

 later. In the later stages the vast majority of the cells in these 

 groups are practically mature erythrocytes lyiiig free in the mesen- 

 chymal spaces in the vicinity of the lymphatics (fig. 11). 



Inasmuch as the reduction in the masses of blood cells in the 

 lymphatics follows the coalescence of the lymphatic spaces to 

 form continuous channels and the establishment of communica- 

 tion between the latter and the jugular lymph sacs, and since 

 the lymph sacs open into the great veins, the blood cells in question 

 eventually reach the general circulation by way of the thoracic 

 duct and jugular lymph sacs. The thoracic duct at one period 

 acts, therefore, as conveyor of the erythrocyte series of hemal 

 cellular elements which have developed from the indifferent 

 mesenchyme along the line of the duct. 



A consideration of the histogenesis of the lymph channels 

 constituting the anlagen of the thoracic duct leads on to contro- 

 versial ground. As stated earlier in this paper, the controversy 

 hinges upon the question whether the lymphatics, exclusive of 

 the lymph sacs or hearts, arise as sprouts or outgrowths from 

 pre-existing vascular channels or de novo from the intercellular 

 mesenchymal spaces. 



It is the opinion of the writer that in the chick the lymph 

 channels which constitute the anlagen of the thoracic duct arise 

 through enlargement and coalescence of intercellular spaces in the 

 mesenchymal tissue, and that the endothelial lining of these chan- 

 nels is derived directly from indifferent mesenchymal cells^ that 

 chance to border upon the spaces. In the material studied there 



^ It should be understood that when 'mesenchymal cells' are spoken of, they 

 are considered as the irregularly stellate masses of protoplasm the processes of 

 which anastomose to form the mesenchymal syncytium, or reticulum. 



