The Organism as a Whole, from a Physicochemical Viewpoint

by Jacques Loeb

2. This influence of temperature upon development has been used to find the condi­tions determining fluctuating varia­tion. The reader knows that by this expression are understood the differences between individuals of a pure strain or breed. These varia­tions are not inherited, a fact contrary to the idea of Darwin, who assumed that by the selec­tion of extreme cases of fluctuating varia­tion new varieties could develop. What is the basis of this fluctuating varia­tion? The writer concluded that if fluctuating varia­tions were due to a slight varia­tion in the quantity of a specific substance—in some cases an enzyme—required for the forma­tion of a hereditary character, the temperature coefficient might be used to test the idea. We have just seen that the time required from insemina­tion until the cell division of the first egg occurs is very sharply defined for each temperature. If a large number e. g. one hundred or more eggs are under observa­tion simultaneously in a microscopic field it can be seen that they do not all segment at the same time but in succession; this is the expression of fluctuating varia­tion. Miss Chamberlain and the writer have measured the time which elapses between the moment the first egg of such a group segments and the moment the last egg begins its segmenta­tion, and found that this latitude of varia­tion is also very definite for each temperature, and that its temperature coefficient is for each interval of 10° practically identical with the temperature coefficient of the segmenta­tion for the same interval.[257] The slight devia­tions are practically all in the same sense and accounted for by a slight deficiency in the nature of the experi­ments. The two following tables give the latitude of varia­tions for different temperatures for the first segmenta­tion in Arbacia and the temperature coefficient for this latitude and the rate of segmenta­tion. These two latter coefficients are practically identical.

TABLE XII

Temperature	Latitude of Variation	Temperature	Latitude of Variation
°C.	Minutes	°C.	Minutes
19	52.5	18	12.0
10	39.5	19	12.5
11	26.0	20	19.6
12	22.5	21	18.0
13	19.2	22	17.8
14	17.5	23	18.0
15	13.0	24	18.0
		25	15.0

TABLE XIII

Temperature Interval	temperature coefficient of
	Latitude of Variation	Segmentation
°C.
19–19	4.2	4.7
10–20	3.9	3.8
11–21	3.2	3.3
12–22	2.8	3.1
13–23	2.4	2.8
14–24	2.3	2.8
15–25	2.6	2.5

If we assume that the temperature coefficient for the segmenta­tion of the egg is that of a chemical reac­tion (other than oxida­tion) underlying the process of segmenta­tion, the fluctuating varia­tion in the time of the segmenta­tions of the various eggs fertilized at the same time is due to the fact that the mass of the enzyme controlling that reac­tion varies within definite limits in different eggs. The first egg segmenting at a given temperature has the maximal, the last egg segmenting has the minimal mass of enzyme. It should be added that the time of the first segmenta­tion is determined by the cytoplasm and is not a Mendelian character, as was stated in a previous chapter.

3. The point of importance to us is that the influence of temperature upon the organism is so constant that if disturbing factors are removed it would be possible to use the time from insemina­tion to the first segmenta­tion of an egg of Arbacia as a thermometer on the basis of the table on page [295].

Facts of this character should dispose of the idea that the organism as a whole does not react with that degree of machine-like precision which we find in the realm of physics and chemistry. Such an idea could only arise from the fact that biologists have not been in the habit of looking for quantitative laws, chiefly, perhaps, because the difficulties due to disturbing secondary factors were too great. The worker in physics knows that in order to discover the laws of a phenomenon all the disturbing factors which might influence the result must first be removed. When the biologist works with an organism as a whole he is rarely able to accomplish this since the various disturbing influences, being inseparable from the life of the organism, can often not be entirely removed. In this case the biologist must look for an organism in which by chance this elimina­tion of secondary condi­tions is possible. The following example may serve as an illustra­tion of this rather important point in biological work. Although all normal human beings have about the same temperature, yet if the heart-beats of a large number of healthy human beings are measured the rate is found to vary enormously. Thus v. Körösy found among soldiers under the most favourable and most constant condi­tions of observa­tions—the soldiers were examined early in the morning before rising—varia­tions in the rate of heart-beat between 42 and 108. In view of this fact, those opposed to the idea that the organism as a whole obeys purely physico­chemical laws might find it preposterous to imagine that the rate of heart-beat could be used as a thermometer. Yet if we observe the influence of temperature on the rate of the heart-beat of a large number of embryos of the fish Fundulus, while the embryos are still in the egg, we find that at the same temperature each heart beats at the same rate, the devia­tions being only slight and such as the fluctuating varia­tions would demand.[258] This constancy is so great that the rate of heart-beat of these embryos could in fact be used as a rough thermometer. The influence of temperature upon the rate of heart-beat is completely reversible so that when we measure the rate for increasing as well as for decreasing temperatures we get approximately the same values as the following table shows.

TABLE XIV