622 



SCIENCE. 



[N. S. Vol. I. Xo. 23. 



is well known, and it is a singular fact that 

 it is only the outer digits, i. e., minimus and 

 poUex, or hallux, or those most exposed to 

 the liability of injury during development, 

 that are, as a rule, duplicated. If the fore- 

 going view is correct, the origin of super- 

 numerary 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 develop- 

 ment is so generally ignored that it must be 

 especially referred to here. I now refer to 

 the statical properties of the germinal sub- 

 stance in modifying development. Some of 

 its effects we have already taken note of 

 above. Karyokinesis has been shown by 

 Hertwig to be dominated by the principle 

 that the plane of division of a cell is alwaj's 

 at right angles to its greatest dimension, a 

 fact readily verified. The greatest dimen- 

 sion of the cell in turn is also often, if not 

 usually, determined by the conditions of 

 free and interfacial surface-tension mani- 

 fested between the members of a cellular 

 aggregate composing a segmenting egg. 

 This appears to have a determining eifect 

 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 have been most exhaus- 

 tively studied by Professor E. G. Conklin, 

 regulate segmentation still remains to be 

 determined. There can, however, be but 

 one explanation of such movements, and 

 that must be a mechanical one, but its nature 

 is entirely unknown. Wilson has shown 

 that the conditions of free and interfacial 



surface-tension in Amjjhioxus varj^ in dif- 

 ferent eggs from some unexplained cause, 

 so that the earlier cleavages of this form- 

 also vary to a corresponding and remark- 

 able degree. In other cases surface-ten- 

 sional forces operate under similar recurring 

 conditions. In the fish-egg I have wit- 

 nessed the reappearance of the same or 

 similar interplay of statical energies thrice 

 in succession, so as to produce three similar 

 successive sets of formal changes in the egg 

 that are traceable to the action of simi- 

 lar statical agencies. In A, Fig. 4, the 



germ a has assiimed a lenticular form of 

 statical equilibrium; after segmentation of 

 the same disk has proceeded some way, as 

 in B, the disk, as a cellular aggregate, has 

 again assumed the lenticular form of equi- 

 librium, while the outermost row of cells, c, 

 are individually in a similar condition of 

 equilibrium. 



These facts are quite sufficient to estab- 

 lish the general truth of the statement that 

 at no stage is the ontogeny of a species ex- 

 empt from the modifjang effect of the sur- 

 face-tensions of its own plasma acting be- 

 tween the cells as if they were so much 

 viscous dead matter. Such statical efi'ects 

 are not overcome at any stage of the devel- 

 opment, or even during the life of any or- 

 ganism. On account of the universal pres- 

 ence and effect of this factor in both the 

 plant and animal worlds, as a modifier of 

 form, we are obliged to consider it as an 

 agent of the first importance in the possible 

 development of the future science of exact 

 dynamical morphology. Its action is so 



