3 88 /• ARTHUR THOMSON [may 



much alike, they do not grow at the same rate, and Mehnert's con- 

 clusion, which he seeks to establish by statistics, is that the rapidity 

 of orowth is proportional to the degree of development finally attained. 

 The lono 1 femur grows most quickly, the short first phalanx of the 

 thumb is slowest of all. The rapidity of the ontogenetic development 

 (growth-process) of an organ is proportional to the height of differentia- 

 tion which is eventually reached. And the rapidity is believed to be 

 on the increase from generation to generation in the case of all structures 

 which are still evolving. 



This acceleration is said to be due to a cumulative inheritance of 

 developmental energy. But we cannot rest here. It must surely 

 admit of further analysis in terms of the conditions favouring cell 

 division, the production of nuclein substances, and so forth. 



Here then we have the author's thesis which claims rank as an 

 induction, and it becomes of interest to hear what he has to say on the 

 general problems of embryology. Even if we do not accept his con- 

 clusions, it is instructive to have the vexed questions re-discussed from 

 a fresh standpoint. The endless references and the ten pages of 

 bibliography show that the author has not been partial in his survey. 



Like His, who first pointed out thirty years ago (1868) that 

 orowth proceeded at different rates in different parts of the embryo, 

 and that the rate of growth increases with the physiological dignity of 

 the structure, Mehnert believes that the conception of organ-forming 

 germinal areas (" organbildende Keimesbezirke ") is inevitable. He 

 supports this by reference to the numerous independent individual 

 variations in rate exhibited in the development of organs, which 

 siu'o-est that each part pursues its own path, now hastening and again 

 la<wino-, and also by reference to Bora's wonderful experiments on 

 crafting tadpoles, where organs separated from their natural neighbour- 

 hood went on developing in a manner certainly suggestive of " self- 

 differentiation." At the same time, Mehnert does not go to the 

 extreme of denying that the development of an organ is influenced by 

 its environment in the widest sense, e.g. by the pressure of an adjacent 

 organ or by gravity. 



In a number of curves and tables, the author gives the results of 

 his measurements of the rate of growth from the beginning of the 

 rudiment of an organ onwards. These all relate to parts of the 

 skeleton, and show (in man) a rapidly ascending curve on to birth, 

 and then a rapid decrease followed in early childhood by a slow rise. 

 It seems to us that the exposition of these measurements, on which the 

 whole point of the book depends, is very inadequate, the curves are 

 badly done and most tedious to decipher. 



He ventures to distinguish the first apex of the curve on to birth 

 as the climax of the preformed character (" Evolutionselevation "), and 

 the second in early childhood as the expression of acquired character 

 (" epigenetische Elevation "), or the result of " functional hypertrophy " 



