8 



Trie Late Blastula in Relation to Certain Innate 

 Pnysiolo^ical Conditions: Tw^innin^ 



A. Introduction 



B. Problem of differentiation 



1. Definition of differentiation; kinds of differentiation 



2. Self-differentiation and dependent differentiation 



C. Concept of potency in relation to differentiation 



1. Definition of potency 



2. Some terms used to describe different states of potency 



a. Totipotency and harmonious totipotency 



b. Determination and potency limitation 



c. Prospective potency and prospective fate 



d. Autonomous potency 



e. Competence 



D. The blastula in relation to twinning 



1. Some definitions 



a. Dizygotic or fraternal twins 



b. Monozygotic or identical twins 



c. Polyembryony 



2. Basis of true or identical twinning 



3. Some experimentally produced, twinning conditions 



E. Importance of the organization center of the late blastula 



A. Introduction 



In the preceding two chapters the blastula is defined as a morphological 

 entity composed of six, presumptive, organ-forming areas — areas which are 

 poised and ready for the next phase of development or gastrulation. How- 

 ever, the attainment of this morphological condition with its presumptive, 

 organ-forming areas is valid and fruitful in a developmental way only if it 

 has developed within certain physiological conditions which serve as a spark 

 to initiate gastrulation and carry it through to its completion. 



The physiological conditions of the blastula are attained, as are its mor- 

 phological characteristics, through a process of diiferentiation. Moreover, 

 during the development of the blastula, different areas acquire different abilities 

 to undergo physiological change and, hence, possess different abilities or 



373 



