160 University of California Publications in Zoology [Vol. 11 



l.ymph or plasma. The transfer in many cases was obviously 

 beneficial, as tissues which were beginning to show signs of 

 deterioration very frequently became more active and presented 

 a more healthy appearance after being placed in the fresh 

 medium. In many preparations the cover slips were raised and 

 replaced every few days so as to allow a change of air in the cell, 

 but it was not discovered that this procedure enabled the cells to 

 thrive any better than in preparations that were not so treated. 



]\Iost of the preparations for the study of epithelium were 

 taken from the tail of the young tadpole. The thin transparent 

 expansion of this organ contains a relatively large amount of 

 epithelium, the thin median portion consisting mainly of con- 

 nective tissue with branching cells embedded in a comparatively 

 firm matrix. The connective tissue elements of such preparations 

 remained relatively fixed in position, the movements of cells 

 being confined mainly to the ectoderm. The ectodermic cells, 

 especially in the older tadpoles, are so easily distinguished by 

 their structure and diffuse granular pigmentation that their 

 recognition does not present the least difficulty. 



Within twenty-four hours after the pieces were mounted, 

 strands of epithelium were to be seen in many cultures extending 

 into the plasma. Around the piece there were also a number of 

 free epithelial cells which had become loosened from their 

 attachment and had wandered out into the fluid. In prepara- 

 tions several days old these free cells were more numerous and 

 scattered over a wider area. Epithelial cells from young em- 

 bryos with but a short stub of a tail are relatively less active 

 than those from older larvae. The reason for this is probably 

 that they are crowded so full of yolk granules that changes of 

 form are considerably impeded. Epithelial cells from the ventral 

 side of young embryos are le.ss active than those from the dorsal 

 side, doubtless because they have more yolk and less protoplasm. 

 Nevertheless the thin protoplasmic outer part of these cells shows 

 active amoeboid movements, putting out fine processes here and 

 drawing them in there, and it is very probably on account of 

 such movements that the cells are able to slowly change their 

 position. In older larvae the yolk in the ectodermal cells gradu- 

 ally disappears and the relative proportion of protoplasm greatly 



