LYMPHATICS IN FROG LARVAE 13 



and runs straight to the base of the tail, gradually tapering 

 to its posterior end. The body cavities are simple and entirely 

 undistended, leaving all inner structures freely exposed to 

 view. The pronephros is prominent anteriorly, lying lateral 

 to the yolk mass, along the ventral borders of the first four 

 muscle segments, just back of the branchial region. 



The blood vessels (first period) 



As has been explained, these stages of frog embryos are 

 extremely difficult to inject, the vessels of the different organ 

 systems being often quite erratic or uncertain in their recep- 

 tion of the injection fluid, which tends to run into the large 

 vessels around a region rather than into the special plexuses 

 of the region itself. The balance of blood pressures or inter- 

 vascular tension between organs being frequently disturbed, 

 an injection may flow either into the vessels of only a single 

 system or irregularly into portions of neighboring organs. 



As in other vertebrate embryos, the early circulation func- 

 tions through a number of plexuses, lying in the various 

 primary organ systems, interconnected to form a complete 

 vascular mechanism in which physiological balance is main- 

 tained through free responses of flow from one region to 

 another as local changes in blood pressure occur. It is natural 

 to expect, then, that a new system, like the lymphatics, would 

 arise as an extension of some part of this preexisting circula- 

 tory mechanism constantly adjusted to its already well- 

 established functions and this, indeed, appears to be the case. 



To insure a complete picture, with correct details of the 

 whole vascular mechanism in the first lymphatic stages, it has 

 been necessary to make many specimens and test the injection 

 in different regions with great care. The results are embodied 

 in figure 1 (and fig. la for ventral view), drawn from the same 

 specimen, and checked against numerous special injections of 

 the same stage. 



These earliest figures are to be studied first for the general 

 arrangement of the blood vessels in relation to the organs in 

 their primary state; following which, the location and charac- 



