ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 343 



this may be in part the result of unfavourable environmental conditions 

 for the cells. Thus, tumour cells degenerate and die when relatively far 

 from the blood-vessels. The fitness of a tissue to grow depends on the 

 specific adaptation between the tissue and the body fluids, and on the 

 way in which various substances are carried to the tissue. The success 

 of transplanted tissue depends on similar conditions. The power of 

 living on is much greater than the power of growing. While the 

 differentiation of such cells as ganglion cells may entail the loss of the 

 power to propagate, and may also entail greater sensitiveness to stimuli, 

 it does not necessarily mean that the cells in question must die. The 

 problem of the prolongation of life may to a great extent depend on the 

 possibility of preventing injurious influences which at present disturb 

 the function of ganglion cells from attacking these cells and causing 

 their death. 



2. The germ cells are potentially immortal, but this cannot be 

 realized without the nuclear changes involved in maturation, followed 

 by fertilization or parthenogenetic development. Similarly, Woodruff 

 and Erdmann have emphasized the importance of endomixis in the 

 generations of Paramecium, which also show potential immortality. Of 

 such rearrangements in somatic cells or in tumour cells there is no 

 evidence, nor of any rhythmic occurrence of vigour and depression. 



:!. Poisons acting on an organism may affect the germ cells, and 

 thus the following generations. Lesions recur, but not always the same 

 lesions. In a similar way in the case of somatic (tumour) cells, 

 characters acquired under the influence of external agencies may be 

 transmitted to the succeeding- cell generations. It can be shown in the 

 case of the somatic cells that apparently similar changes produced through 

 different external agencies are really not identical, but specific. It is 

 suggested that such a specificity of transmitted characters may also exist 

 in the case of germ cells, despite the apparent identity of changes pro- 

 duced through different external agencies. Loeb reports on a case in 

 which a change in somatic cells was transferable to the following cell 

 generations. It was of such a nature that it would be pronounced non- 

 specific, if ordinary criteria are used. But by the use of special methods 

 the change was shown to possess a definite characteristic relationship to 

 the external factor that caused it, and must therefore be called specific. 

 In germ cells there may also exist a difference in the mode of produc- 

 tion of lesions through the action of different substances, and con- 

 sequently aspecificity in the acquired lesion, notwithstanding the 

 apparently unspecific character of the lesion. 



Development of Nerve Cells of the Chick.* — E. V. Cowdry has 

 studied the development of the cytoplasmic constituents of the nerve 

 cells of the chick, with particular reference to mitochondria and neuro- 

 fibrils. The neurofibrils are first formed in developing chick embryos 

 as a differentiation of the ground substance (in the majority of cases of 

 the peripheral neuroblasts), at a stage of fifteen somites, 5*8 mm. in 

 length, after forty hours' incubation at 39° C. The earliest neurofibrils 



Amer. Journ. Anat., xv. (1914) pp. 3S9-428 (5 pis.). 



2 B 2 



