CELLULAR DIFFERENTIATION AND EXTERNAL 



ENVIRONMENT 



By C. M. CHILD 



DEPARTMENT OP ZOOLOGY, THE UNIVERSITY OF CHICAGO, CHICAGO, ILL., AND SCHOOL 

 OF BIOLOGICAL SCIENCES, STANFORD UNIVERSITY, CALIF. 



As long as the biologist remained wholly 

 or chiefly an observer of organisms, living 

 or dead, he was often so impressed by their 

 constancy of spatial and chronological 

 order and pattern that he inclined to re- 

 gard them as in some way predetermined 

 and independent of environmental factors. 

 Even after the concept of specific vital 

 energy was discredited and the principle 

 of evolution was established, predetermi- 

 nistic theories were still advanced and 

 widely held. Development was often con- 

 ceived as a sort of construction period with 

 heredity as the constructor. When con- 

 struction had proceeded to a certain stage 

 the organism began to function in relation 

 to an external world. It is of interest that 

 Willhelm Roux, who played so important a 

 role in the application of experiment to 

 problems of animal development, held es- 

 sentially this view. 



Development of the experimental method 

 in the field of zoology and animal physi- 

 ology, largely within the past fifty years, 

 has played no small part in altering our 

 conceptions of living things. As experi- 

 mentation has extended its field and has 

 become more accurate and analytic, we 

 have found it increasingly difficult to 

 separate organism and environment. It 

 has become evident that we know living 

 things, from the cell to man, only in rela- 

 tion to an environment, either organismic 

 or external, and cannot conceive them apart 

 from that relation. "We are now pretty well 

 agreed that, so far as the individual is 

 concerned, what we have called heredity is 

 the sum total of potentialities of structure 

 and function; that is, of behavior in the 

 broadest sense, implicit in the physicochemi- 

 cal constitution of the protoplasmic system 

 from which that individual develops; that 

 for realization of any of these potentialities 



as the structure and function of an indi- 

 vidual in development, certain relations to 

 environment, either of the parent organism 

 or external environment, are essential ; and 

 that with different environments different 

 potentialities are realized. Here we are 

 concerned only with questions of realization 

 of potentialities in actual development and 

 in relation to an external environment. 



Considering the cell as a living indi- 

 vidual, we find as the most general charac- 

 teristic of its pattern a difference of some 

 sort between surface and interior, a sur- 

 face-interior pattern. The protoplasm ad- 

 joining the external medium is usually 

 visibly, as well as physically, and probably 

 to some extent chemically, different from 

 the interior. The so-called plasma mem- 

 brane is perhaps in some cases no more 

 than a monomolecular or micellar film with 

 molecules or particles definitely oriented in 

 relation to the surface; from this it ranges 

 to a visible morphological surface layer. 

 It is not, however, a dead covering of the 

 cell, but is as much alive as any other part. 

 Beneath this an ectoplasm or cortex may 

 differ in structure and behavior from 

 deeper regions. In unicellular organisms, 

 morphological differentiation is limited to 

 the ectoplasmic or cortical layer. That sur- 

 face-interior pattern originates in direct 

 relation to external environment appears 

 evident. The transformation of entoplasm 

 into ectoplasm by exposure to the external 

 medium in consequence of section is a char- 

 acteristic occurrence in Protozoa and many 

 other cells. There is apparently a trans- 

 formation in both directions in amoeboid 

 movement. In some animal eggs, notably 

 those of ctenophores and ascidians, the ecto- 

 plasm becomes so stable that it retains its 

 distinctive characteristics even though re- 

 moved from the surface by developmental 



67 



