52 HENRY MCELDERRY KNOWER 



The earliest of these injections in the submaxillary region 

 (illustrated in fig. 20) reveals a simple system in the area 

 centered in a well-defined tubular lymph duct, nearly as large 

 as the transverse vein which it parallels. The duct begins 

 laterally in terminal processes of the temporal sinus and ex- 

 tends across the ventral area to the mid-zone occupied by the 

 thyroid gland. A long slender tube, the circum-oral (Co.L.) 

 lymphatic, runs forward lateral to the mouth from the junc- 

 tion of the main submaxillary duct with the temporal sinus 

 (Tpl.S.) (figs. 19, 20, 21, 22). The wall of the mid-section of 

 the main duct expands ventrally in irregular pouches which 

 form the first phase of the 'central submaxillary sinus' 

 (CN.S.). Distally, the duct ends in a plexiform tubular tip 

 bearing small pointed processes on the wall (fig. 20). A 

 strong projection, with similar characters, directed forward 

 toward the mouth, will later give rise to the sac of the man- 

 dibular sinus of this side (the tubular character of the end 

 section of the duct and its relation to the central sinus rudi- 

 ment are seen from a different angle in fig. 21). It should be 

 noted that both this sinus rudiment and the circum-oral lym- 

 phatic lie deep to the sucker. The pericardial lymphatic is 

 shown in the figure (fig. 20) lying ventral to the external 

 jugular vein, with its swollen tip in contact with the posterior 

 wall of the central duct. It appears not to communicate with 

 the cavity of the duct. The exact relations of the two at this 

 stage cannot, however, be determined, though the pericardial 

 lymphatic (Pc.L.) is apparently injected from behind through 

 its origin in the proximal end of the jugular duct. 



The injection of the primary tubular lymph duct in the sub- 

 maxillary region from the jugular lymphatic immediately 

 following the arrival of the distal end of this vessel (temporal 

 sinus) in the ventral area indicates that this submaxillary 

 lymph duct is a local end-product of the general progress of 

 the centrifugal growth of the lymphatic system through the 

 larva, like other terminal ducts invading peripheral areas in 

 this stage. In the absence of a preformed system of lym- 

 phatics in the region, and judging from the characters and 



