186 PROFESSOR MARSHALL AND EDWARD J. BLES. 



INTRODUCTION. 



The development of the blood vessels, and more especially of the 

 aortic arches, in Amphibians is a subject of peculiar interest, owing 

 to the fact that members of this group repeat in their individual 

 history the transition from aquatic to aerial respiration, which there 

 is every reason to regard as ancestral, not merely for themselves, 

 but for all the higher Vertebrates as well. 



If we are right in interpreting individual development as repre- 

 senting, in however modified a form, the past ancestral history of 

 the race, then the study of the respiratory change in Amphibians 

 ought to throw an important light on the mode in which the 

 transition first came about. Moreover, the actual change is in 

 Amphibians a gradual one, the gills and lungs being used simul- 

 taneously for a considerable period of time, or in many forms 

 throughout life, so that there is abundant opportunity for studying 

 quietly and deliberately the details of the transformation. 



In spite, however, of the interest of the problem, and of the ease 

 with which suitable material for investigating it may be obtained, 

 there are many points in actual development that are still very 

 imperfectly understood, and it was in the hope of clearing up some 

 of the doubtful or disputed matters that the investigations were 

 undertaken, the results of which are recorded in the present paper. 



For material we have employed almost exclusively embryos and 

 tadpoles of the common frog, Hana temporaries; including all the 

 more important stages from the first appearance of the blood vessels 

 to what is practically the adult condition. With regard to methods 

 of investigation, the examination of living specimens has been of 

 considerable use ; but all our more important results were obtained 

 from dissections and from the study of sections. Of the latter a 

 very large series was prepared, specimens of all the stages being cut 

 in the three principal planes, — transverse, horizontal, and sagittal. 

 For hardening embryos, we have found Kleinenberg's picro-sulphuric 

 ftcid, and corrosive sublimate the most useful reagents ; for staining 



