AITIIOH s AMSlUAtT OK THIS PAPEI! ISSUED UV 

 THE lUHI.HXIHAPHIC SEBVICE, DECEMBER 29 



STUDIES ON THE GROWTH OF BLOOD-VESSELS IN 

 THE TAIL OF THE FROG LARVA— BY OBSERVATION 

 AND EXPERIMENT ON THE LIVING ANIMAL 



ELIOT R. CLARK 



■' Department of Anatomy, University of Missouri 



SIXTEEN FIGURES 



These studies were begun and part of them were made in the 

 laboratory and under the inspiration of my beloved teacher and 

 master, the late Professor Franklin P. Mall, and it is with a 

 sense of the deepest gratitude and reverence that I acknowledge 

 the immeasurable debt which I owe to him. 



INTRODUCTION 



The development of the vascular system falls broadly into 

 two stages: (1) the stage of primary differentiation, or histogen- 

 esis, and (2) the stage of extension and elaboration of arteries, 

 veins, and capillaries. The exact^ manner and place, in which 

 the primary differentiation occurs is an unsettled problem, and 

 is, at the present time, the subject of spirited controversy. It 

 has not been satisfactorily decided whether blood-vessel en- 

 dothelium differentiates from entoderm, or mesoderm — and if 

 from mesoderm, whether from mesenchyme generally or from 

 the mesothelial lining of the coelom. Nor has it been deter- 

 mined whether this primary differentiation occurs on the walls of 

 the yolk sac alone, or in the embryo proper, or whether it may 

 take place both on the yolk sac and in the embryo. Another 

 unsettled point is the extent of time over which the primary 

 differentiation takes place. Recent discussions and observa- 

 tions supporting one or another of these views may be found in 

 the following: Minot ('12), Evans ('12), Ruckert and Mollier 

 ('06), Schulte ('14), Bremer ('14), Stockard ('15 A),Reagan ('17), 

 Sabin ('17). 



The second stage of vascular development includes the further 



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THE AMERICAN JOURNAL OF ANATOMY, VOL. 23, NO 1 , 



