THORACIC DUCT IN THE CHICK 143 



being subjected to pressure and friction incident to the flow of 

 the interstitial substance. Thus there arises in the mesenchyme 

 a space larger than the original interstitial spaces but derived 

 directly from them by their enlargement and coalescence (cf. figs. 

 12, 13 and 14). So far as there is any definite lining for the new 

 space, it is formed by the partiall}^ or wholly flattened mesen- 

 chymal cells upon which the fluid in the space impinges (fig. 13). 



The further changes in one of these larger spaces consists in 

 the main of its elongation through the enlargement and addition 

 to it of other intercellular mesench3aiial spaces, a progressive 

 flattening of the cells along its sides, and an approximation of 

 the edges of the flattened cells to form a definite lining of endo- 

 thelium. There is thus formed a distinct channel in the mesen- 

 chyme. For the most part it is lined and its lumen is separated 

 from the surrounding interstitial mesenchymal spaces by a layer 

 of endothelial cells which represent metamorphosed stellate ele- 

 ments of the mesenchyme (figs, 15 and 16). At or near its ends 

 the channel opens freely into the adjoining mesenchymal tissue 

 spaces which in the further course of development are added to 

 the lumen of the channel and thus become a part of it (figs. 15, 

 16, 17 and 18). The endothelial lining already formed merges 

 near the ends of the channel with the mesenchymal syncytium 

 which in turn, as the channel elongates, gives rise to more endo- 

 thelium by differentiation of certain of its elements (figs. 15, 16, 

 17 and 18). 



As stated in a previous paragraph, the larger spaces appear 

 in several localities in the mesenchyme. Consequently the chan- 

 nels resulting from their further development are for a time iso- 

 lated; that is, they are not directly connected with one another 

 or with the jugular lymph sacs or any part of the hemal vascular 

 system. These isolated spaces and channels constitute the mul- 

 tiple anlagen of the thoracic duct (fig. 21), 



In succeeding stages each of the channels in question increases 

 in size, especially in length, until it meets and coalesces with its 

 neighbors. The increase in length is due in the main to the addi- 

 tion of more mesenchymal tissue spaces to its ends and the con- 

 comitant transformation of more stellate mesenchvmal cells into 



