142 ADAM M. MILLER 



is no evidence of any growing out, budding or sprouting from the 

 endothelium of pre-existing blood vessels, the tissue in which 

 the thoracic duct develops being non-vascular. The lymphatics 

 in question, and their endothelial lining, arise independently of 

 pre-existing vascular channels. 



Prior to the appearance of any specialized spaces or channels 

 in the mesenchyme in the region of the future thoracic duct, the 

 mesenchymal syncytium consists of irregularly stellate proto- 

 plasmic elements the slender processes of which anastomose freely 

 with like processes of neighboring elements. The cytoplasm is 

 finely granular and the nuclei, while relatively large and vesic- 

 ular, contain little chromatin and one or two distinct nucleoli. 

 Among the protoplasmic components of this tissue are the cor- 

 respondingly irregular interstices or spaces which also are con- 

 tinuous with one another. These are called the mesenchymal 

 intercellular spaces. The tissue as a whole might be compared 

 with a sponge, the anastomosing protoplasmic parts representing 

 the parenchyma of the sponge and the intercellular spaces the 

 pores. While some of the protoplasmic elements during this 

 time differentiate into blood cells, as previously described, our 

 conception of the general syncytium and its spaces is in no way 

 invalidated. 



The first changes in the mesenchyme leading toward the for- 

 mation of definite channels occur during the latter half of the 

 sixth day of incubation. These changes, instead of involving 

 the mesenchyme generally, begin in several localities. In one of 

 the localities, for example, the intercellular spaces increase in 

 size and coalesce, most of the protoplasmic elements of the syn- 

 cytium being pushed farther apart or broken. In this manner a 

 considerably larger space is formed out of a number of the original 

 smaller mesenchymal spaces (fig. 9, 17). For the most part the 

 smaller spaces of the surrounding tissue open freely into the larger 

 space, although in places along the edge of the latter the proto- 

 plasmic elements lose some of their stellate character and become 

 flattened on the side toward the larger space. 



The phenomena in general indicate the accumulation of the 

 fluid filling the mesenchymal spaces, the cells and their processes 



