56 ELIOT R. CLARK 



the circulation of blood cells commences, and a new circulating 

 capillary has been added to the system. This whole process 

 may, however, not be completed, for some sprouts grow out a 

 short distance and are retracted, while some in which the lumen 

 has been formed, never have a circulation, but retrogress — becom- 

 ing solid, and disappearing. Throughout this process the endo- 

 thelium remains complete, the lumen being separated from the 

 tissue fluid outside by a complete investment of endothelium. 



The facts concerning the morphological changes which take 

 place in the formation of sprouts are clear enough; the question 

 then arises as to why sprouts are sent out, to what sort of stimu- 

 lus the endothelium responds when it sends out a sprout. The 

 answer to this question is not entirely clear, yet certain facts to- 

 gether with certain general considerations justify the proposal 

 of an hypothesis. 



A study of the positions at which sprouts are formed and of the 

 general direction taken in their growth shows that they are pre- 

 ceded in their formation by the growth of the other tissues and 

 that they extend into regions where the amount of tissue not 

 yet vascularized is greatest in amount. In the tad-pole's tail, 

 at early stages, vessels develop first along the muscle — the thick- 

 est part of the tail. Later they grow out into the fin expansions, 

 which attain a considerable size before vessels reach them. 

 Growth of new capillaries continues in a general direction toward 

 the dorsal and ventral margins of the fin, until eventually the 

 plexus reaches nearly to the margin. During the growth of this 

 first set of vessels, the fin remains thin, and the capillaries — save 

 for the thickest part next the muscle — are all in a single plane. 

 Later, the fin becomes much thicker and there occurs a corre- 

 sponding new growth of capillaries, from the older parts of the 

 plexus, which pass toward the epidermis, and form plexuses in 

 two new planes. 



In both cases it is clear that the growth of new blood-capillar- 

 ies has been secondary to the growth of the outside tissue. It 

 has been suggested by Thoma as an hypothesis that the stimu- 

 lus responsible for sprout formation lies in an increase in blood- 

 pressure. If this wxre so, one would expect to find them growing 



