46 CHARLES F. W. McCLURE 



embryo is superficially located beneath the integument and oc- 

 cupies an extensive area in the territory ventral to the mouth 

 (see Kampmeier, '15, Plate 1). It communicates on each side of 

 the head with the lateral cephalic lymph sinus which is also super- 

 ficially located, and through which it now drains into the lateral 

 pharyngeal lymphatic (ductus cephalicus) and thence on through 

 the anterior lymph-hearts into the veins. 



The above observations leave no room for doubt that a continuous 

 lymph flow is now established for the first time between the large 

 superficial lymph sinuses of the head and the veins. From the 

 conditions observed on the fifth day, this drainage of the head 

 sinuses into the veins must have been initiated in Buf o sometime 

 between the fifth and sixth days. 



The lymph from the posterior regions now drams into the ante- 

 rior lymph-hearts by superficially located lymphatics. One of 

 these, the lymphatic of the lateral line, as in the trout embryo 

 (McClure, '15), forms the direct caudal continuation of the lateral 

 pharyngeal lymphatic (ductus cephalicus). Definite valves are 

 now present for the first time between the anterior lymph-hearts and 

 the veins and between the anterior lymph-hearts and the lateral 

 pharyngeal lymphatic. 



A few blood-corpuscles were often found in the anterior lymph- 

 hearts which entered from the veins before valves were formed. 

 A similar condition has been observed by the writer ('15, p. 11) in 

 the embryo of the trout. 



Coagulated lymph and an occasional typical mononuclear 

 leucocyte are found in the large lymph sinuses of the head. 

 Mononuclears are also found occasionally in the pronephros and 

 mesenchyme. Many free cells in the mesenchyme, circulation 

 and pronephros are, however, still in an active state of division. 

 It is only essential to mention here in this connection, that blood- 

 cell development is more advanced than on the fifth day, and that 

 some of the free cells resemble what one would regard as typical 

 mononuclear leucocytes. It would appear that the presence of a 

 few typical leucocytes at this time, coincides with the opportunity 

 presented to them of ingesting waste products of metabolism, 

 which may now be brought to them by the lymphatics from the 

 intercellular tissue spaces. 



