44 BIOLOGICAL LECTURES. 



and of their transmission by inheritance to offspring there is also 

 no doubt. These facts make it probable at any rate that re- 

 generation of distal parts and the likelihood with which they 

 reappear in duplicate, is due to causes similar or identical in 

 character with those that lead to the production of double 

 monsters, by shaking, mutilation or other physical interference 

 with the normal development of the oosperm. The question 

 of the inheritance of mutilations is consequently far from being 

 concluded as viewed from this new standpoint. Much evidence 

 might be adduced in support of my contention did space allow. 

 The hereditary transmission of such monstrosities as supernu- 

 merary digits is well known, and it is a singular fact that it is 

 only the outer digits, i.e., minimus and pollex, or hallux, or 

 those most exposed to the liability of injury during develop- 

 ment that are, as a rule, duplicated. If the foregoing view is 

 correct, the origin of supernumerary digits is not always to be 

 ascribed to reversion. It must not be understood, however, 

 that the theory is here defended that mutilations effected after 

 adolescence is reached are likely to be transmitted. 



The " mutilations " here referred to are hardly to be regarded 

 as such, but rather as the results of mechanical interference or 

 disturbance of the statical equilibrium of those parts of the 

 developing germ that are duplicated, as we see, in obedience to 

 the principle discovered by Barfurth. 



Another dynamical factor in development is so generally 

 ignored that it must be especially referred to here. I now 

 refer to the statical properties of the germinal substance in 

 modifying development. Some of its effects we have already 

 taken note of above. Karyokinesis has been shown by Hert- 

 wig to be dominated by the principle that the plane of division 

 of a cell is always at right angles to its greatest dimension, a 

 fact readily verified. The greatest dimension of the cell in 

 turn is also often, if not usually, determined by the conditions 

 of free and interfacial surface-tension manifested between the 

 members of a cellular aggregate composing a segmenting egg. 

 This appears to have a determining effect upon the plan of the 

 cleavage. How far and in what way the remarkable move- 

 ments of the centrosomes that occur during cleavage, and that 



