378 THE BLASTULA IN RELATION TO INNATE CONDITIONS 



development. Potency limitation, however, is not always the result of the dif- 

 ferentiation process. For instance, in the development of the oocyte in the 

 ovary, the building up of the various conditions, characteristic of the totipotent 

 state, is a feature of the differentiation of the oocyte. 



The word determination is applied to those unknown and invisible changes 

 occurring within a cell or cells which effect a limitation or restriction of potency. 

 As a result of this potency limitation, differentiation becomes restricted to a 

 specific channel of development, denoting a particular kind of cell or structure. 

 Ultimately, by the activities of limiting influences upon the resulting blasto- 

 meres during cleavage, the totipotent condition of the mature egg becomes 

 dismembered and segregated into a patchwork or mosaic of general areas of 

 the blastula, each area having a generalized, presumptive, organ-forming po- 

 tency. As we have already observed, in the mature chordate blastula there 

 are six of these major, presumptive organ-forming areas (five if we regard 

 the two mesodermal areas as one). By the application of other limiting in- 

 fluences during gastrulation or the next phase of development, each of these 

 general areas becomes divided into minor areas which are limited to a potency 

 value of a particular organ or part of an organ. The process which brings 

 about the determination of individual organs or parts of organs is called 

 individuation. 



When potency limitation has reduced generalized and greater potency value 

 to the status of a general organ system (e.g., nervous system or digestive 

 system) with the determination (i.e., individuation) of particular organs 

 within such a system, the condition is described as one of rigid or irrevocable 

 determination. Such tissues, transplanted to other parts of the embryo favor- 

 able for their development, tend to remain limited to an expression of one 

 inherent potency value and do not give origin to different kinds of tissues or 

 organs. Thus, determined liver rudiment will differentiate into liver tissue, 

 stomach rudiment into stomach tissue, forebrain material into forebrain 

 tissue, etc. 



In many instances determination within a group of cells is brought about 

 because of their position in the developing organism and not because of in- 

 trinsic, self-differentiating conditions within the cells. Because their position 

 foreordains their determination in the future, the condition is spoken of as 

 positional or presumptive determination. For example, in the late amphibian 

 blastula, the composite ectodermal area of the epiblast will become divided, 

 during the next phase of development, into epidermal and neural areas as a 

 result of the influences at work during gastrulation, especially the activities 

 of the chordamesodermal area. Therefore, one may regard these areas as 

 already determined, in a presumptive sense, even in the late blastula, although 

 their actual determination as definite epidermal and neural tissue will not 

 occur until later. 



As stated in the preceding paragraphs, determination is the result of potency 



