44 CHAELES F. W. McCLURE 



tial, in order for the dye to reach the typical cells at such a time, 

 for circulatory conditions to be favorable for the transference of 

 dye from the intercellular tissue spaces to the general circulation. 

 We should therefore examine the circulatory system of the earlier 

 larval stages to determine when such conditions might first be 

 found. The earliest stage of development in the toad which 

 would permit of the transference of lymph by the lymphatics 

 from the periphery to the veins, has been found to occur, approxi- 

 mately, between the fifth and sixth days after fertilization. We 

 may therefore consider in some detail the general ontogenetic 

 conditions which prevail at this tune. 



On the fifth day after fertilization, the ventral and lateral 

 superficial lymph sinuses of the head are represented in Bufo by 

 a number of independent and disconnected anlages or endothelial- 

 lined vesicles or sacs. These anlages neither communicate with 

 each other, with the veins nor with any other lymphatics at this 

 time. This particular stage of their development has been well 

 illustrated by Kampmeier in the toad ('15, fig. 12). The anterior 

 lymph-hearts are present and are situated just caudal to the 

 pronephros. These lymph-hearts are often filled with blood-cor- 

 puscles and, as no valves are present, they are in wide-open com- 

 munication with the veins. A lymphatic opens dorsally into the 

 lymph-heart which can be traced craniad for only a short distance 

 at this stage. This lymphatic also often contains blood-corpuscles 

 as no valves between it and the lymph-heart have been formed. 

 It has been designated by Hoyer ('08) as the ductus cephalicus 

 and corresponds to the lateral pharyngeal lymphatic in the 

 embryo of the trout (McClure, '15). 



On the fifth day after fertilization no lymph can possibly reach 

 the anterior lymph-hearts or general circulation from the lymph 

 sinuses of the head region, because the sinuses have not estab- 

 lished a connection with the lateral pharngeal lymphatic (ductus 

 cephalicus) and do not otherwise communicate with the veins. 

 No blood corpuscles of any character were found in the cephalic 

 lymph sinuses on the fifth day, which is also significant of the fact 

 that lymph or blood does not circulate through them at this time. 



