374 E. L. CLARK AND E. R. CLARK 



t§nth day, the hearts grow more rounded in shape and become 

 surrounded by a layer of fat. With the gradual reduction of 

 the curve of the vertebral column, they eventually occupy a 

 more dorsal and anterior position with reference to the os ilii, 

 Sala found that the lymph hearts communicate with the inter- 

 segmental coccygeal veins — at first connecting with the first five 

 of these veins, and, in later stages, with three of them. 



From his study of the lymph heart in cross sections, Sala found 

 that, in embryos of seven and one-half days, its walls are made 

 up of mesenchyme which penetrates the lumen of the sac in the 

 form of trabeculae. In nine-day chicks, muscle fibers begin to 

 differentiate in this connective tissue, and eventually they form 

 a layer in direct contact with the endothelial lining. In embryos 

 of thirteen days the muscle fibers and the endothelium are sep- 

 arated by a layer of mesenchyme. 



Sala also investigated the earlier stages of the lymph heart, 

 in cross sections of chicks six and one-half to seven days old. 

 He found a row of irregular spaces connected with the first five 

 intersegmental coccygeal veins and concluded that the lymph 

 heart was formed by the flowing together of these spaces. 



Mierzejewski in 1909 studied the superficial lymphatics of 

 chick embryos by injection. He confirmed the results of Sala 

 as to the development of the lymph heart but states that the 

 anlage of the lymph heart first appears in chicks of five and 

 one-half days, instead of six and one-half days, as Sala had 

 stated. Mierzejewski agrees with Sala as fro the time of appear- 

 ance of the muscle fibers in the wall of the lymph heart and 

 states that, at nine days, the Ijaiiph heart begins to pulsate and 

 beats once to every eight or ten contractions of the blood heart. 



The authors have studied this region in the stages of embryos 

 investigated by Sala and Mierzejewski and also in earlier stages. 

 We have found that the spaces in the mesenchyme, described by 

 Sala as the anlage of the lymph heart, are in reality definite 

 enlarged lymphatic capillaries, which form a continuous plexus 

 connected with the intersegmental coccygeal veins, and whose 

 endothelial lining can be demonstrated easily by injection with 

 dilute silver nitrate, and also by the microscopic study of sections. 



