JrxE 7, 1895.] 



SCIENCE. 



625 



course of many generations of individuals. 



This variation may be directed by the 

 concurrence of a series of natiu-al conditions 

 operating dynamically (natural selection). 

 Or, interbreeding and crossing, with care 

 or under Nature, may unite by means of 

 reciprocal integration (fertilization) two 

 molecular mechanisms whose total struc- 

 ture and sum when thus united, as in sex- 

 ual reproduction, may vary by the mere 

 combination of the two dynamical sj'stems 

 (egg and sperm), diflering slightly from one 

 another in potentiality. Finally, adaptive 

 changes may be called forth dynamically in 

 the internal -structure of such developing 

 reciprocally integi-ated systems that must 

 be traced back to changes in the mechanism 

 of metabolism of the parent as well as in 

 the germs it gives off. Such changes pro- 

 duced in the germ must become visible in 

 the effects they produce, as transmitted for- 

 mal changes exhibited in the course of de- 

 velopment. 



The tendency or trend of development, 

 however, of a given form must be pretty 

 constant, and controlled within compara- 

 tively narrow limits by the initial adult or 

 attained structure. That is, what has been 

 attained must formally affect that which is 

 to be attained in future. Tliis is the idea 

 that underlies the VervoUlcommnungs-Princip, 

 principle of perfecting, of Nageli. This 

 view also tacitly recognizes the theory of 

 change of function proposed by Dohrn, as 

 well as the theories of substitution, super- 

 position and epimorphosis of Kleincnberg, 

 Spencer and Haacke. Once a condition of 

 stable equilibrium has been reached in the 

 series of transformation of the molecular 

 mechanism represented bj' the germ, dur- 

 ing the development of an organism, we may 

 have what Eimer has called Genepista.^i'i, re- 

 .sulting in the fixity or stabilitj^ of an or- 

 ganic species, under stable conditions. 



The cell is a complete organism, but it 

 loses its physiological and moi-phological 



autonomy when comltined with other cells. 

 We may regard the nucleus, cytoplasm and 

 centrosome as reciprocally related parts ; 

 one of them not much more important than 

 the others. The observed behavior of the 

 centrosome would indicate, as Verworn has 

 held, that it is the important agent in cellu- 

 lar metabolism. If this is true, metabolism 

 has certain centers in the cell to and from 

 which molecular transformations are ef- 

 fected rythmically in every direction, with 

 the centrosome as focal points. This view 

 agrees perfectly with the facts, since the 

 raj's of the asters may be regarded as the 

 morphological expression of a dj-namical 

 process of intermolecular diflusiou due to 

 metabolism, as Kijlliker has suspected (Ge- 

 webelehre, 6th ed.). 



Such a process would not only sei-ve to 

 alter the surface and interfacial-tensions of 

 the cells during ontogeny, but also vary the 

 osmotic pressure within them. Conse- 

 quently, we maj' conceive that all of the 

 phenomena of development, including the 

 appearance and disappearance of cavities 

 within a germ by changing conditions of 

 osmosis, may receive a dynamical explana- 

 tion. The centrosomes may, moreover, be 

 conceived to lie at the foci of very complex 

 material figures, the lioundaries of which 

 are finite equipotential cellular surfaces. 

 These focal points are clearly near or within 

 the nuclei. The equipotential surfaces de- 

 veloped by the sorting or readjusting pro- 

 cess that goes on during segmentation in 

 order continually and rj'thmically to re- 

 store the dj-namicale(iuilibrium of the molec- 

 ular germinal aggi'cgate as a mechanically 

 constructed sj-stem during life and develop- 

 ment, through gi'owth and metabolism, 

 must maintain the shapes of organisms as 

 we see them. The epigenetic theory of in- 

 heritance therefore promises us a seciire 

 basis upon which to found a theorj'' of the 

 mechanics of development, as well as a 

 theorj' of the origin of morpliological types. 



