May 14, 1915] 



SCIENCE 



709 



treatment with butyric acid or by alkali, 

 and if the eggs are afterwards prevented 

 from developing (by putting them for a 

 certain length of time into sea water con- 

 taining NaCN) they will go back into a 

 resting condition from which they can be 

 aroused again by a treatment with sperm. 

 We suspect that in this case the reversion 

 in development is also accompanied by a 

 reversion in the rate of oxidations. 



We see then that our definition of a cell 

 as being constantly ready to grow and seg- 

 ment is not strictly fulfilled even in the 

 case of the egg cell, which, according to 

 Weismann, we may consider as immortal. 

 Instead we see that the egg cell can appar- 

 ently alternate between a resting condition 

 and an active condition, and that the na- 

 ture of the cortical layer of the egg deter- 

 mines in which of the two conditions the 

 egg exists. 



From this we might conclude that our 

 original definition, that each cell will grow 

 and multiply eternally, may hold after all 

 if we add the fact, that in the egg cell a 

 variation in the nature of the cortical 

 layer may start or inhibit cell division and 

 growth. We may next ask : Does this addi- 

 tion also satisfy the facts we find in the 

 adult body where the cells come to rest un- 

 less they are called into active growth again 

 by a wound or by the not definitely known 

 causes of tumor formation? Or, in other 

 words: Is it only a change in the cortical 

 layer which condemns the cells of the adult 

 body to rest and those of the young body 

 to grow? 



Unfortunately, our task is not so easy. 

 The unfertilized egg which is ready for 

 fertilization will die comparatively rapidly, 

 unless it is fertilized by sperm or treated 

 by the methods of artificial partheno- 

 genesis. We can prolong its life by sup- 

 pressing its oxidations. Before the egg is 

 mature its duration of life seems longer. 



If the eggs of the starfish are allowed to 

 mature they die in a few hours if not fer- 

 tilized; if they are prevented from becom- 

 ing mature they live much longer. It is 

 not known that anjrthing similar to this 

 exists in the somatic cells of the adult 

 animal. Until such knowledge is acquired 

 we must be prepared to admit that the rest- 

 ing cell of an adult organism is in a condi- 

 tion which is not comparable to that of the 

 unfertilized egg. 



We know that the growth of resting cells 

 in a body may be induced if the blood con- 

 tains certain substances which differ for 

 different kinds of cells. One of the most 

 recent and most striking observations in 

 this direction was that of Gudematseh, 

 who found that in the tadpole of a frog or 

 a toad, whose legs usually do not begin to 

 grow until it is several months old, the 

 legs can be induced to grow out at any 

 time, even in very young specimens, by 

 feeding them with the substance from 

 thyroid glands. No other material seems 

 to have such an effect. The thyroid con- 

 tains iodine, and Morse states that if instead 

 of the gland iodized amino-acids are fed 

 the same result can be produced. We must 

 draw the conclusion that the normal out- 

 growth of legs in a tadpole is also due to 

 the presence in the body of substances 

 similar to the thyroid in their action (it 

 may possibly be thyroid substance) which 

 is either formed in the body or taken up 

 with the food. 



That the phenomena of larval meta- 

 morphosis are independent of the influence 

 of the central nervous system has been 

 amply demonstrated. Thus I could show 

 in 1896 that if we cut through the spinal 

 cord of an amblystoma larva the meta- 

 morphosis of the body in front and behind 

 the cut takes place simultaneously. Uhlen- 



