GROWTH OF BLOOD-VESSELS IN FROG LARVAE 47 



included in the present study was presented at the meeting of 

 the Am. Ass. of Anat. in 1914 (E. R. Clark ('15), ) where draw- 

 ings were shown. 



DESCRIPTION or FINDINGS 



When the blood-vessels in the dorsal fin of r. sylvatica larvae 

 first become clearly visible, owing to the absorption of some of 

 the yolk and pigment present in young larvae, they form a sys- 

 tem of capillary loops, making an irregular meshwork of rather 

 wide vessels, all connected with one another. On the arterial 

 side they are connected with the main caudal artery, and on the 

 venous side with the main caudal vein, which are located ventral 

 to the notocord, and between the two layers of myotomes. The 

 vessels reach the dorsal fin by passing dorsally between the 

 notocord and spinal cord in the center and the layers of myo- 

 tomes on either side. With the low power of the microscope 

 their course may be easily follow^ed from the main caudal vessels 

 to their emergence ^ from between the myotomes. With the 

 higher power this is more difficult, and in most of the studies 

 made, only the vessels in the dorsal fin proper, after their emer- 

 gence from between the myotomes, are drawn. 



While in many of the studies all the vessels iu the dorsal fin 

 have been followed, a small area is selected for closer study, 

 and for reproduction, because any section illustrates the funda- 

 mental principles involved in blood-vessel development. In the 

 series which is reproduced an area was chosen which included an 

 arteriole and a venule and the region between the two, as well 

 as a part of the regions on either side. This area is sufficiently 

 large to make it possible to follow the changes introduced by 

 the development of new capillaries on the vessels already present. 



The changes which occur in such a selected area are shown in 

 figures 1 to 8, and will now be taken up in detail and analyzed. 



There is present in the first record a very simple type of cir- 

 culation. An arteriole, or, perhaps better, an arterial capillary 

 is seen toward the left. Two branches are given off from this 

 vessel on the right, and two on the left, through which blood 

 corpuscles are circulating. In addition there is a third branch 



