x = 1·128.
That is to say, between the intervals of 10° and 20° C., if it take m days, at a certain given temperature, for a certain stage of development to be attained, it will take m × 1·128n days, when the temperature is n degrees less, for the same stage to be arrived at.
Fig. 29. Calculated values, corresponding to preceding figure.
Fig. [29] is calculated throughout from this value; and it will be seen that it is extremely concordant with the original diagram, as regards all the stages of development and the whole range of temperatures shewn: in spite of the fact that the coefficient on which it is based was derived by an easy method from a very few points in the original curves. {117}
Karl Peter[147], experimenting chiefly on echinoderm eggs, and also making use of Hertwig’s experiments on young tadpoles, gives the normal temperature coefficients for intervals of 10° C. (commonly written Q10) as follows.
| Sphaerechinus | 2·15, |
| Echinus | 2·13, |
| Rana | 2·86. |
These values are not only concordant, but are evidently of the same order of magnitude as the temperature-coefficient in ordinary chemical reactions. Peter has also discovered the very interesting fact that the temperature-coefficient alters with age, usually but not always becoming smaller as age increases.
| Sphaerechinus; | Segmentation | Q10 | = 2·29, |
| Later stages | ″ | = 2·03. | |
| Echinus; | Segmentation | ″ | = 2·30, |
| Later stages | ″ | = 2·08. | |
| Rana; | Segmentation | ″ | = 2·23, |
| Later stages | ″ | = 3·34. |
Furthermore, the temperature coefficient varies with the temperature, diminishing as the temperature rises,—a rule which van’t Hoff has shewn to hold in ordinary chemical operations. Thus, in Rana the temperature coefficient at low temperatures may be as high as 5·6: which is just another way of saying that at low temperatures development is exceptionally retarded.