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A. M. MILLER 



size from the smallest capillary to vessels as large as or larger than 

 the precardinal vein (fig. 4, 5 and fig. 5, 5) . For the most part the 

 vessels are intercommunicating, but a few islands lie in the periph- 

 eral region. The walls of the channels are composed merely of 

 endothelium, without visible condensation of mesenchyme around 

 them (fig. 5, 5). A great majority of the channels are empty; 



Fig. 4 Diagram drawn from a reconstruction of the veins, veno-lymphatics 

 (prelymphatics) and nerves in the cervical and upper thoracic regions of a chick 

 embryo of six days. Right side. 1, Precardinal vein; 2, postcardinal vein; 3, 

 duct of Cuvier; 5, veno-lymphatic (prelymphatic) plexus, the forerunner of the 

 jugular lymph sac; 9a, brachial plexus; 14, subclavian vein. 



a few situated peripherally in the group, contain small numbers 

 of nucleated red blood cells. 



The conditions met with in this stage resemble very closely the 

 conditions in a 10 mm. cat embryo (Huntington and McClure, 

 fig. 49), with the noteworthy exception that in the chick the 

 plexus has already lost its connection with the cardinal veins. 



