DEVELOPMENT OF THE JUGULAR LYMPH SAC 489 



dissolution of the contiguous portions of the two walls. There 

 is no indication of valve formation at the tap, as has been shown 

 to be the case in the mammal. The absence of valves guarding 

 the opening between the lymph sac and the vein may be due to 

 the fact that the tap had just been established in the 8-day-and- 

 14-hour chick. Valve formation may be a subsequent process, 

 as it actually is in the mammal, but further examination of later 

 stages will be necessary to settle this point. The nature of the 

 communication or tap at this stage is shown in fig. 9, 5a, on the 

 right side, and in fig. 10, 5a] its position is shown in fig. 8 by the 

 dotted oval 5a. 



The wall of the sac is composed of endothelium and a thin 

 layer of mesenchyme (fig. 10). No smooth muscle cells can be 

 determined in this layer, although that may be due to the fact 

 that they are insufficiently differentiated by this m.ethod of stain- 

 ing. Mierzejewski states that he has observed rhythmical con- 

 tractions of the anterior lymph heart or sac in embryos of eight 

 days. 



At its caudal end the lymph sac is joined by the thoracic duct 

 (fig. 8, 13). The above mentioned tap, from its position, is in 

 all probability homologous with the jugulo-subclavian tap in the 

 mammal (cf. fig. 62, 16 mm. cat embryo, Huntington and Mc- 

 McClure). In the chick at this stage a second tap is lacking, 

 but it is not improbable that a study of later stages will reveal 

 a homologue of the common jugular tap in the mammal. Accord- 

 ing to the descriptions and figures of the anatomy of the lym- 

 phatics in birds, a branch of the lymphatic trunk from the head 

 and neck, which occurs as a variant on the right side, opens into 

 the jugular or the precaval vein. There is good reason to assume 

 that there exists a considerable variability in the communications 

 between the lymphatic trunks and the veins, as in the cat, and 

 that a careful investigation of the anatomical relations in the adult 

 bird will serve to bring them into accord with the conditions in 

 the mammal. 



A lymph sac in the adult bird has not been described or figured, 

 so far as the writer is aware. However, the conditions in the 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 12, NO. 4 



