520 



SCIENCE 



[N. S. Vol. XLIII. No. 1111 



plant cells where only the superficial layers 

 of the protoplasm show a definite persistent 

 morphology receives a simple interpretation 

 from this point of view, while from any 

 other standpoint it is difficult to find a rea- 

 son for this superficial appearance of order. 

 The superficial origin of the nervous sys- 

 tem in development is perhaps the most 

 notable case in point. 



It is not necessary, however, to assume 

 that every organic individual arises directly 

 through the differential action of environ- 

 mental factors. When the metabolic gra- 

 dients with their associated protoplasmic 

 conditions are once determined in a mass of 

 protoplasm or cells they or their proto- 

 plasmic substratum may persist for many 

 generations through division or other re- 

 productive processes. In other cases factors 

 in the intra-individual environment may 

 determine the gradient or gradients in cer- 

 tain parts. The polarity of the egg, for 

 example, shows in most cases in both ani- 

 mals and plants a definite relation to the 

 point of attachment of the growing egg cell 

 to the parent body, and there is good rea- 

 son to believe that the differential action of 

 the egg's environment in the organism 

 determines its polarity. In some cases, how- 

 ever, this polarity, if present, is apparently 

 eliminated and a new polarity established 

 by external factors acting after isolation, 

 as for example in the egg of the alga Fucus, 

 where the axis of the egg and so of the plant 

 is apparently determined by incident light, 

 or in its absence by other differential rela- 

 tions to external conditions. Evidently a 

 physiological axis may be inherited through 

 one or more generations after it is once 

 established, or it may be determined de 

 novo in each reproduction. Experiment 

 demonstrates that even in many cases where 

 it is inherited in nature we can eliminate it 

 and determine the establishment of a new 

 axis by the differential action of external 

 conditions. 



Considering for a moment another point, 

 the question may be raised whether a mere 

 gradient in rate of metabolism with its 

 correlate of protoplasmic condition is ade- 

 quate to account for the differentiation 

 that arises along an axis in development. 

 To those who have been accustomed to 

 postulate a great number of qualitatively 

 different entities as the starting point of 

 the organic individual such a conception 

 may seem to be almost ridiculously inade- 

 quate. The facts, however, are these. We 

 can produce experimentally morphological 

 differences which are clearly qualitative 

 through the action of external factors, such, 

 for example, as temperature, which act on 

 metabolism primarily in a purely quanti- 

 tative way. Moreover, it is clear from vari- 

 ous lines of evidence that the character of 

 the substances which accumulate in a par- 

 ticular protoplasm as components of its 

 structural substratum is very closely as- 

 sociated with metabolic rate. When the 

 rate is high only certain substances pro- 

 duced in the course of the metabolic reac- 

 tions and which are relatively stable under 

 these conditions can accumulate as a struc- 

 tural substratum, while other substances 

 are broken down and eliminated. With a 

 lower metabolic rate some of these other 

 substances do not break down so readily 

 and therefore they also may accumulate 

 and so on. Take the simple case of the ac- 

 cumulation of fat in a cell. We know that 

 a low metabolic rate favors fat accumula- 

 tion and a higher rate may lead to its dis- 

 appearance. But we can not doubt that 

 after the accumulation of fat in a cell has 

 begun, the presence of the fat alters the 

 metabolic processes occurring in that cell: 

 its appearance in the cell is associated 

 with a certain metabolic rate, but once 

 present it may alter not merely the rate, 

 but the kind of metabolism which occurs. 

 Various factors indicate also that differ- 



