CHAPTER XVI 



ORIGIN AND NATURE OF EMBRYONIC PATTERNS 

 THE PROBLEMS AND THE EVIDENCE 



OUESTIONS of the origin and nature of pattern or "organization" 

 of the animal egg have been the subject of discussion and specu- 

 lation since the beginnings of embryological investigation. This 

 has been due in part to the fact that the ovarian oocyte is inaccessible to 

 most present experimental procedure but perhaps also in some measure 

 to the very general belief that embryonic development is of primary 

 significance and that all other types of development have only a secondary 

 interest. If we attempt to interpret embryonic pattern and development 

 in terms of what experiment has shown concerning the simpler forms of 

 development, we may perhaps at least come nearer agreement concerning 

 certain questions of embryonic pattern. 



Preceding chapters have dealt chiefly with experimental evidence con- 

 cerning the characteristics of the spatial or regional pattern of individual 

 development, and the question of the origin of pattern under natural 

 conditions has received little attention. The earliest distinguishable pat- 

 tern appears to consist of regional differences of some sort on a molar or 

 morphological, rather than a molecular or micellar, scale. Protoplasms 

 are complex colloidal and crystalloidal physicochemical systems, and or- 

 ganisms appear to be systems of quantitatively or qualitatively different 

 protoplasms. The series of changes constituting what we call "living" 

 take place in many kinds of protoplasms, but an organism is an integra- 

 tion in an orderly and definite pattern of different rates or kinds of living. 

 The question of the origin and nature of this pattern is of fundamental 

 importance for our conception of the organism and of development. 



If developmental pattern is primarily independent of environment, 

 either it must be continuously present as a spatial pattern through all 

 the cell divisions from one generation to another, or it must originate 

 autonomously in the cell or cell mass concerned. The assumption of an 

 "intimate structure" of some sort, adequate, ex hypothesi, as a basis for 

 pattern, is only a statement of the problem in terms of structure. The 

 Roux-Weismann theory of qualitative nuclear division states the prob- 



644 



