Mehnert's Principles of Development. 



By Pkofessor J. Arthur Thomson, M.A. 



It is well known that the rapid development of embryology has been 

 mainly along morphological lines ; of the physiology of development 

 we know extremely little. For a time the gap was filled by 

 generalities — -laws of growth, principles of development, polarities, 

 and the like, in fact a plethora of vague speculation. Then came a 

 stern reaction, a franker confession of ignorance, and a renewed 

 enthusiasm for concrete research, as illustrated for instance in the 

 recent boom of experimental embryology. Mehnert's work 1 is not 

 experimental, but it abounds in measurements, and it is this that 

 forms its attraction, for the beginning of measuring is often the 

 beginning of science. 



Two years ago Dr. Ernst Mehnert published an essay entitled 

 " Kainogenesis," in which he gave the results of a comparative study 

 of 384 extremities of various embryos from turtle to ostrich, and 

 formulated certain general embryological conclusions. Far from 

 accepting the prevalent vague idea of uniformity in development, he 

 showed that each type or even species has its individuality as 

 regards the order in which various parts appear and the rate at 

 which they develop. He gave many illustrations proving that the 

 time at which a structure appears in the individual development 

 cannot be taken as indicative of the historical or phylogenetic age 

 of that structure. In this regard he showed clearly that the familiar 

 generalisation " ontogeny is a short recapitulation of phylogeny " must 

 not be taken too literally. Not that our author doubts the famous 

 biogenetic law ; he only points out that in applying it care must be 

 taken to appreciate the effects of varying rates of development in 

 different creatures. It is this notion of time - displacements in 

 development which has been elaborated in the second essay on the 

 principle of organogenesis. In other words, the author seeks to show 

 the importance of the developmental changes which may result from 

 changes in the growth-rate of particular parts. 



1 " Biomechanik erschlossen aus clem Principe der Organ ogenese." By Dr. Ernst 

 Mehnert. 8vo, pp. viii. + 177, with 21 tigs. Jena: Gustav Fischer, 1898. Price 5 Marks. 



385 



