52 ELIOT R. CLARK 



iceably larger, and the vessels longer. The last record shows 

 this enlargement of the tail most markedly. ' The tail has in- 

 creased not only in length and height, but also in thickness, and 

 with the enlargement there has been a very great development 

 of new capillaries, in the widened spaces of the blood-vessel 

 meshwork. In the half of the fin next the muscle, where the 

 growth in thickness has been most pronounced, many of the 

 new capillaries are in new planes, more superficial than the 

 earlier formed vessels. As regards the fate of the vessels present 

 in the earlier stages, it is seen that there has been a marked dif- 

 ferentiation. In figure 1 the vessels present are nearly all of a 

 uniform diameter. In each successive record there is a progres- 

 sive differentiation, in which certain capillaries increase in size, 

 others remain of the same, or slightly diminished caliber, while 

 others disappear. In the last stage this differentiation is seen 

 at its maximum; definite arterioles and venules have formed, 

 which supply and drain considerable capillary areas. In this 

 elaborated system there are present many of the same vessels 

 and parts of vessels which w^ere present in the first stage recorded. 

 Some have been incorporated as parts of the larger vessels, others 

 are still capillaries, while others have disappeared. 



Considered as a whole, then, this series shows strikingly that 

 arterioles and venules develop, at least in this region in tad- 

 poles, not by a steady outgrowth of a single vessel, which grows 

 straight ahead into a new region, giving off branches where 

 they are needed, and fulfilling its predetermined destiny to grow 

 in a particular place, but rather by the sending out of numerous 

 capillaries, in various directions, which anastomose, adding new 

 loops of circulating capillaries to those already present. Of these 

 new loops some are so placed that a circulation is never estab- 

 lished through them, and they disappear; others are incorporated 

 as parts of arteriole or venule or remain as capillaries. The ef- 

 fect of the addition of new capillaries on the system already 

 present depends upon the relation' which the older parts bear 

 to the new; thus a vessel which is at one stage the chief vessel of 

 the region may entirely disappear, while another vessel, w^hich 

 is small, and has a slow circulation at one stage, may later be a 



