DEVELOPMENT OF THE JUGULAR LYMPH SAC 485 



along the dorsal aspect of the precardinal vein, and is almost 

 equal in caliber to the vein itself (fig. 6, 5). This structure, 

 which is the immediate forerunner of the jugular lymph sac, 

 although it does not at this stage communicate with the venous 

 system, is the result of the enlargement and coalescence of the 

 numerous smaller channels of the preceding stages. While the 

 sac as a whole is relatively straight, its wall is exceedingly irreg- 

 ular. This irregularity is due to diverticula which represent 

 channels not yet completely incorporated in the sac. A few iso- 

 lated channels also are situated in the immediate vicinity of the 

 sac. 



As noted above, the sac at this stage does not in any way com- 

 municate with the venous system. Consequently it is properly 

 spoken of as a prelymphatic. It comprises an endothelial bag 

 which lies free in the mesenchyme, and is practically destitute of 

 blood cells, a few being present in some of the diverticula (fig. 

 7, 5). 



There can be no doubt that the veno-lymphatic forerunner 

 of the jugular lymph sac loses connection with the venous system, 

 and remains free for a considerable period. This disconnection 

 is shown to be absolute in the seven day chick, where the wall of the 

 prelymphatic sac and the wall of the vein do not even approxi- 

 mate (fig. 6, 5 and fig. 7, 5). In the six day chick the plexiform 

 group of channels is free from the vein except, as already noted, 

 for one capillary communication near the cephalic end. Exam- 

 ination of intermediate stages, from which no reconstructions 

 were made, also showed the lack of communication. The first 

 instance of a new connection was observed in an embryo of eight 

 days and fourteen hours. This temporary disconnection between 

 the anlage of the lymph sac and the veins in the chick coincides 

 with the similar condition in the mammal. For example, in a 

 cat embryo of 10.7 mm. (Huntington and McClure, fig. 59, also 

 page 294), to which the stage in the chick under consideration 

 corresponds most closely, the empty prelymphatic sac is not in 

 communication with the venous system. Observation thus far, 

 however, goes to show that the disconnected condition is probably 

 of longer duration in the chick than in the mammal. 



