CELLULAR DIFFERENTIATION AND EXTERNAL ENVIRONMENT 



71 



plasm of an egg, or iu a certain cell layer 

 of an embryo. 



In the earlier stages of axiate develop- 

 mental pattern differences in rate of de- 

 velopmental activities are very generally 

 evident to direct observation at different 

 levels of the polar axis and often of other 

 axes. Graded differences in rate of res- 

 piration, rate of reduction of vital dyes 

 in low oxygen, susceptibility to external 

 agents, and other lines of experimental evi- 

 dence, all agree in showing the presence 

 of quantitative physiological gradients as 

 characteristic features of such pattern in 

 many organisms. In axiate pattern in its 

 simplest terms, as in buds, certain recon- 

 stitutions of isolated pieces or cell aggre- 

 gates, we can find no evidence in early 

 stages of anything more than these quanti- 

 tative gradients, and it is possible to estab- 

 lish new patterns by means of quantitative 

 gradients of environmental factors. Ap- 

 parently specific differences in substance 

 and metabolism arise gradually at different 

 levels of such physiological gradients. In 

 a system so complex as a living protoplasm, 

 differences in concentration of oxygen, nu- 

 tritive substances, water, ions, the various 

 products of synthesis and catabolism, dif- 

 ferences in the colloid substrate, and in- 

 terrelations of different levels along a 

 gradient initiated as purely quantitative, 

 provide ample basis for the origin and 

 development of specific or qualitative dif- 

 ferences along its course. In fact, it is 

 difficult to believe that such a gradient, 

 even though primarily quantitative in 

 origin, remains so for any considerable 

 period of development; but it does appear 

 that quantitative differences may remain 

 predominant for a longer or shorter time 

 during development. The gradual prog- 

 ress of so-called determination and of dif- 

 ferentiation of parts along an axis suggests 

 the gradual origin and development of 

 qualitative chemical differences in relation 

 to an earlier pattern in which such differ- 

 entiation has not occurred. It may also be 

 pointed out that a gradient may be an 

 essential factor in determining activation 

 of different hereditary potentialities, or 



genes, in different cells at different levels 

 of it. 



The assumption that developmental pat- 

 tern in its simplest terms is always to be 

 found in the egg is at least misleading: it 

 may or may not be the case. In some 

 animal eggs there is at present no evidence 

 of anything more than quantitative grad- 

 ient pattern at the beginning of embryonic 

 development, but complete proof that noth- 

 ing more is present is lacking. Other eggs 

 show various degrees of regional cytoplas- 

 mic organization or differentiation, though 

 a quantitative pattern may also still be 

 present. It does not follow, however, that 

 the differentiation of the egg is an essential 

 or primary feature of the developmental 

 pattern of the species concerned. The as- 

 cidian egg, for example, shows a consid- 

 erable degree of cytoplasmic organization, 

 but this apparently represents a stage in 

 development of pattern in the egg cell 

 rather than a fundamental feature of as- 

 cidian developmental pattern. An ascid- 

 ian can develop from buds of different 

 origins, from pieces of stolen or body iso- 

 lated by section, and from the cell aggre- 

 gates often called winter buds. What we 

 know at present concerning these forms of 

 development affords no ground for believ- 

 ing that all or any of them have patterns 

 similar to that of the egg at the beginning 

 of embryonic development, but it is evident 

 that all of them acquire in some way the 

 essentials of ascidian pattern, and those 

 essentials appear to be gradient patterns 

 originating in the relations of the systems 

 in which development occurs to their or- 

 ganismic or extraorganismic development. 

 Only by experimental analysis and com- 

 parison of the patterns of the different 

 forms of development, their beginnings, 

 and their relations to environmental fac- 

 tors can we hope to distinguish the funda- 

 mental factors of developmental pattern of 

 a species from the features incidental to 

 a particular form of development. 



According to species, stage and form of 

 development, region of body, relation to 

 other parts and to external environment, 

 the developmental fates of particular cells 



