'46 ELIOT R. CLARK 



there is, as the foregoing review and discussion shows, differ- 

 ence of opinion sufficient to justify further observation and 

 experiment. 



METHODS USED IN PRESENT STUDIES 



Since most of the studies referred to were made on successive 

 stages, usually of injected embryos, in fixed preparations, it 

 seemed that it would be worth while to study the changes in the 

 same vessels of the same living embryo, following certain vessels 

 through the critical stages in their development, keeping rec- 

 ords of the circulatory conditions, and of all changes in the size 

 of the vessels, and the direction of the angle of branching, et 

 cetera. For such a study the transparent fin expansion of the 

 tail of frog larvae is admirably adapted, for a larva can, by the 

 use of chloretone as an anesthetic, be kept under observation 

 over a period of weeks, and careful camera lucida records made 

 as frequently as desired. Since the chloretone interferes but 

 little with the heart beat, records can also be kept of the circu- 

 latory conditions in each of the vessels which is being watched. 

 (For details of the method used see E. R. Clark ('12).) In the 

 most extensive series of studies made on a single tad-pole, the 

 observations were started when the larva (rana sylvatica) first 

 became transparent enough to enable the course of the vessels 

 in the dorsal fin to be made out, and records were made at daily 

 intervals, at first, when new formation of vessels was most rapid, 

 later, when changes were slower, at considerably longer intervals. 

 During the observations the larva increased in length from 10.5 

 mm. to 29 mm. There was thus procured a record giving the 

 vascular changes, with notes as to the condition of circulation 

 in each vessel for a considerable section of the fin, from a stage 

 at which the entire system consisted of a few capillary loops, to 

 a stage in which a fairly complicated system of arterioles, capil- 

 laries and venules had developed. In addition to this series of 

 studies, numerous shorter studies were made, on larvae of r. 

 sylvatica, r. palustris and r. catesbiana. Brief reference has 

 been made in an earlier paper (E. R. Clark ('09) ) to the blood- 

 vessel changes in the tail of the frog larva, and some of the matter 



