DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 257 



of this. Shortly before reaching the aorta, the fourth efferent vessel 

 gives off the pulmonary and cutaneous arteries, whose course and 

 relations are as before. 



5. General Considerations. 



"We have been somewhat puzzled at this stage by our failure in 

 some cases to find the direct connections between afferent and efferent 

 vessels which appear to be constantly present at an earlier stage. 

 At present we are unable to explain our results in this respect, 

 unless there should prove to be great individual variability in the 

 precise time and mode of appearance of these communications in the 

 several arches. It is in some cases a matter of extreme difficulty 

 to determine from sections whether such a communication does or 

 does not exist ; and in some specimens, whose appearance suggests 

 in the strongest manner that the direct connection is present, Ave 

 have been unable to trace it out in a satisfactory and conclusive 

 manner. To follow with certainty a single tortuous capillary passage 

 through half-a-dozen or more sections, in which it may be cut in all 

 conceivable planes, is not always possible. 



Boas*, who has investigated these later stages by injecting the 

 vessels and then dissecting them out, has found direct connections 

 present between the afferent and efferent vessels of the first and 

 second branchial arches in a tadpole in which the tail had begun to 

 shrink, but was still long : the third and fourth arches were un- 

 fortunately destroyed in the specimen. The communications in the 

 first and second arches were so free that the injection mass passed 

 across from the base of the afferent to the efferent vessel, leaving the 

 dorsal part of the afferent vessel empty. 



IX. THE CONDITION OF THE VESSELS OF THE BRANCHIAL 

 ARCHES IN YOUNG FROGS TOWARDS THE END OF THE 

 FIRST YEAR. 



This is the last stage that we propose to deal with. The frogs, 

 which are now several months old, have lost their gills completely, 

 though remnants may persist in a function] ess condition. There is 



* Boas, "Ueber den COIIUS ArterioauH und di<; Aortwibngon dcr Ampbibien," 

 " Uorphologucbefl Jahrbuch," vii., 1881, p. 547. 

 S 



