78 ELIOT R. CLARK 



ten Gesetze, welches das Wachstum der Gefasswand von den Stro- 

 mungsverhaltnissen des Blutes abhangig erscheinen lasst. 



At another place, however, (p. 32) he suggests that the de- 

 velopment of large vessels in the site of the two primitive aortae 

 may be due to favorable mechanical or nutritional conditions. 



That other main vessels develop, in chick embryos, before 

 circulation commences, has been shown by Miss Sabin ('17) 

 and referred to earlier in this paper. 



In order to study the character of the blood-vessels in the tail 

 fin of frog embryos deprived of a circulation, the type of plexus 

 formed, and the mode of growth, I have employed Knower's 

 method on frog larvae, and studied the vessels in the fin expan- 

 sion of the tail. The heart was removed after it was sufficiently 

 developed to be clearly visible, under the binocular microscope, 

 but before it had started to beat, by making an opening through 

 the skin, into the pericardial cavity grasping the heart with a 

 pair of forceps, and dissecting it loose with a needle. In some 

 cases a small pulsating fragment was left but there was no blood- 

 circulation. Embryos operated on in this way rarely live more 

 than seven days, if the weather is warm, though they survive for 

 ten to twelve days in cool weather. As Knower had described 

 them, they become greatly swollen, due to the accumulation of 

 fluid in the body cavities. They are very active, swim about 

 the dish restlessly, and respond quickly to stimulation. 



Unlike Knower, I found a considerable vascular development in 

 the tail fins. Since the tail is opaque in early stages, due to the yolk 

 and pigment, it was not possible to obtain very striking records 

 of the growing blood-vessels. In several cases, however, the tail 

 became sufficiently clear to permit records to be made. In one 

 case records were made covering three days of growth of the 

 vessels in the dorsal fin, and a considerable amount of growth 

 was observed. The blood-vessels in the dorsal fin, in these lar- 

 vae without hearts, form a primitive close-meshed plexus, of 

 delicate vessels. In some cases the vessels are distended with 

 blood cells, in others they are distended with a clear fluid, while 

 in others they are very narrow. The blood cells are pushed 

 about in the vessels by the movements of the embryos, and are 



