360 ELIOT R. CLARK 



out from the trunk at exactly the spot chosen. A new drawing 

 was made of the various structures (fig. 3) . The larva was kept 

 under anesthesia the entire day, and frequent drawings made, 

 figs. 3, 4, 5, 6, 7 and 8, for the lymphatic was in a state of very 

 rapid growth. A record was made of the mesenchyme cells well 

 in advance of the growing lymphatic, toward the edge of 

 the fin. The following day a new record was made (fig. 9,) in 

 which the area of recorded mesenchyme cells was extended to 

 the fin margin. From this time on, for thirty-one days, records 

 were made almost daily, and in each record there was incl ded 

 lymphatic, caudal vein, and all the mesenchyme cells, with their 

 main processes, in the region selected. It is unnecessary to re- 

 produce all the drawings. They were demonstrated at the meet- 

 ing mentioned. Two later stages are selected (figs. 10 and 11) 

 which were drawn at eight and at twenty days later than fig. 9. 

 In addition to this series, numerous other studies were made on 

 other regions in this same tadpole, as well as on other larvae. 

 Fig. 15, for instance, in which are given the results of a study 

 primarily of the changes in the capillary wall, is taken from the 

 dorsal fin of another larva. The camera lucida drawings were 

 made at an enlargement of 450x, and corrections were made at 

 a still higher enlargement. 



In order to simplify the description, the two tissues — mesen- 

 chyme and lymphatic — will be taken up separately. The relations 

 between the two as they grow together will then be indicated. 



The mesenchyme cells at the time at which they first become 

 visible in the fin expansion of the larva of hyla pickeringii, consist 

 of an irregularly shaped thick central portion containing a 

 nucleus which is not clearly visible during life. From this central 

 portion extend a varying number of branched processes of differ- 

 ent lengths, and of a thickness which diminishes from center 

 to periphery. The branches in turn give off large numbers of 

 minute fibrillae, which form a richly anastamosing network ex- 

 tending from epidermis to epidermis. These minute fibrillae may 

 be seen and followed in the living larva, by careful focussing. 

 They usually appear as minute dots which seem to travel up or 

 down as the microscope is focussed. When any of these tiny 



