August 18, 1916] 



SCIENCE 



223 



call them individuals. But no two descend- 

 ants of either compound or unicellular or- 

 ganisms are strictly alike. Each maintains 

 an individuality of its own different from 

 that of its immediate forerunners. This 

 diversity must also be accounted for. 



The differences between parents and off- 

 spring are adequately explained by the de- 

 tails of maturation. "Why, however, do the 

 units derived from the fertilized egg differ ? 

 This question is the inevitable consequence 

 of our inability to consider more than one 

 thing at a time. As yet we have neither 

 reckoned with the differential distribution 

 of cytoplasmic substances nor with the inti- 

 mate history of the chromosomes during 

 and after division. 



Students of embryology are familiar 

 with the distribution of "organ-forming" 

 substances. These have been convincingly 

 traced in a number of eggs (Conklin) . The 

 remarkable homologies found in the early 

 development of molluscan and annelidan 

 eggs of various types can be understood 

 only as expressions of the accuracy with 

 which these materials manceuver. 



The visibility of an "organ-forming" 

 substance is the merest accident. In the 

 egg or cell from which an individual comes 

 there may be and probably are materials 

 whose accurate but uneven distribution 

 during cleavage has not been noticed. Ob- 

 viously there may be many occasions on 

 which the cytoplasmic composition is 

 changed during development. 



Differential localization of itself indi- 

 rectly increases the possibilities of further 

 differentiation. With increase in the num- 

 ber of cells come purely physical and me- 

 chanical disturbances of equilibrium. In 

 the readjustments that follow, changes of 

 relation, themselves certain to influence the 

 greatest variety of subsequent events, are 

 inevitable. A crisis like gastrulation can 



not but affect, directly or indirectly, every 

 cell in the system. 



I am not forgetting the work of the 

 Drieschian school of experimentalists. They 

 have sinned abundantly in this field for 

 the origin of two or four individuals from 

 an egg whose blastomeres are separated at 

 the appropriate moment by no means 

 demonstrates a harmonious equipotential 

 system. Harmonious it probably is, but 

 equipotentiality is proved by meridional 

 divisions only to those who consider them 

 identical with equatorial or latitudinal 

 cleavages. The production of viable organ- 

 isms from blastulae has been misinterpreted 

 in the same way. 



Differentiation may also be nuclear in 

 origin. Not only are we unable to exclude 

 the possibility of qualitative and quantita- 

 tive disparities in ordinary mitosis, but we 

 know positively that differences in nuclei 

 may come about after division. We should 

 recall the somatic cells of Ascaris and es- 

 pecially the differential growth of chromo- 

 somes. 



As Conklin has pointed out 7 the chro- 

 matin mass does not necessarily double 

 with each doubling in the number of cleav- 

 age cells, since growth is not shared pro- 

 portionately by all the chromosomes. This 

 fact, which very likely does not apply to 

 the divisions of the sex cells, has been ob- 

 served in the mitoses of early development, 

 divisions which have been but little studied 

 in detail. Such diminutions in the relative 

 sizes of chromosomes may be accompanied 

 by changes in the chromosomal balance 

 and, through this, bring on changes of 

 equilibrium among cytoplasmic processes. 

 Some chromosomes may, in one respect or 

 another, become ineffective, or in their 

 altered circumstances may have effects 

 qualitatively different from their earlier 

 ones. 



