1913] Holmes: Ectodermic Epithelium of Tadpoles 167 



count showed a further increase on the third day. but after four 

 or five days the increase in number was very small. Transferring 

 the cells to fresh media failed to cause more than a very slight 

 increase in number. The possibility suggested itself that by 

 taking cells from young embryos and transferring them at inter- 

 vals into fresh media a continuous culture might be carried on 

 for some time. Whether through inadequate technique or for 

 some other reason this hope was not realized, despite many careful 

 experiments. In many preparations the cells remained to all 

 appearances healthy for three months. In the cells from com- 

 paratively young embryos there was little evidence of progressive 

 change. The yolk content remained apparently the same in these 

 cells for over two months, and the cells showed but little activity, 

 although the}' were quite normal in appearance. Beyond making 

 one or two divisions the isolated ectoderm cells from young 

 embryos failed to undergo any well-marked developmental 

 changes. 



Older ectoderm cells which had used up most of their yolk 

 were more active, showing marked changes in form and especially 

 flattening out upon solid surfaces. The older cells that were 

 isolated and counted failed to give more than a slight increase 

 in number. The cilia in many cases kept beating for several 

 weeks. Some of the cells that had become greatly flattened out 

 showed in a few cultures various stages of amitotic division of 

 the nucleus. In different cells the nucleus was constricted from 

 a slight indentation to an almost complete division, and several 

 cells were found which contained two clearly separate nuclei. 

 No convincing evidence could be obtained, however, that the 

 amitotic division of the nuelei;s was followed by ii division of the 

 cytoplasm. 



Harrison (1912) has observed the differentiation of isolated 

 cells from frog embryos, and I have been able in several cases to 

 confirm his findings. But, so far as my rather limited experi- 

 ence goes, isolated single embryonic cells especially of the ecto- 

 derm, do not differentiate nearly so well as those forming a part 

 of an organized piece of tissue. Several times I have observed 

 pieces of embryos increase in size, become more transparent, and 

 show transformations of the internal parts; and along with 



