80 ELIOT R. CLARK 



also moved about to some extent by the action of gravity. Thus, 

 vessels which have been empty, have a little later been found to 

 be packed with cells, and vessels filled wdth cells, may later be 

 quite empty, or filled only with a clear fluid. Figure 16 shows 

 some of the vessels in the dorsal fin of such a larva, on three differ- 

 ent days. 



On comparing the three records, it is obvious that growth has 

 taken place by the formation of sprouts, which are at first nar- 

 row threads, but which later acquire a lumen. Anastomoses form 

 between neighboring sprouts, and no additions are made by out- 

 side cells. In fact the vascular plexus extends in essentially the 

 same manner as in the normal tail. There is, however, a 

 larger number than in the normal, of fine solid processes. In this 

 particular specimen the vessels are very narrow, and contain no 

 blood cells. 



It is to be noted that the vessels, once formed, show no tendency 

 to differentiate further — into arterioles or venules. All diminish 

 somewhat in caliber. In other specimens all the vessels are con- 

 siderably distended. While three days is not a sufficient time, 

 even in the embryo with a circulation, for much differentiation, 

 still, in stages as early as this, at least a beginning differentiation 

 is noticeable, as may be seen in the series shown in figures 1 to 3. 



It is of interest to note the growth of Ijrmphatics in the embryos 

 deprived of blood circulation. Knower's observation that the 

 anterior lymph hearts in such embryos are larger and beat more 

 strongly than in normal embryos, was confirmed. In the tail 

 fin, lymphatics grow out often beyond the bloodvessels, al- 

 though in the normal embryos at this stage and in this species 

 (Rana pipiens) the blood-vessels in this region grow out well in 

 advance of the lymphatics. The lymphatics are somewhat 

 wider than in normal embryos. The mode of growth of lym- 

 phatics is the same as in normal embryos, by the extension 

 outward of sprouts, and there is no tendency for the lymphatic 

 and blood- vascular endothelium to form anastomoses with one 

 another. 



The enlarged caliber of the lymphatics is of interest, especially 

 in connection with the enlarged lymph hearts, and with the ob- 



