376 THE BLASTULA IN RELATION TO INNATE CONDITIONS 



tiguous cells to realize a specific potency. On the other hand, the presumptive, 

 neural plate region at this time is dependent upon the inducing influence of 

 the chordamesodermal cells during the process of gastrulation for its future 

 realization as neural tissue. This area has little inherent ability to differentiate 

 neural tissue and is described, therefore, as being in a state of dependent 

 differentiation (Roux). Furthermore, the presumptive skin ectoderm (i.e., 

 epidermis), if left alone, will proceed to epidermize during gastrulation, but 

 foreign influences, such as transplantation, into the future neural plate area 

 may induce neural plate cells to form from the presumptive skin ectoderm 

 (fig. 183). The differentiation of neural cells from any of the ectodermal cells 

 of the late blastula thus is dependent upon special influencing factors applied 

 to the cells from without. 



C. Concept of Potency in Relation to Differentiation 



1. Definition of Potency 



The word potency, as used in the field of embryology, refers to that prop- 

 erty of a cell which enables it to undergo differentiation. From this viewpoint, 

 potency may be defined as the power or ability of a cell to give origin to a 

 specific kind of cell or structure or to various kinds of cells and structures. 



h is questionable, in a fundamental sense, whether potency actually is 

 gained or lost during development. It may be that the expression of a given 

 kind of potency, resulting in the formation of a specific type of cell, is merely 

 the result of a restriction imposed upon other potentialities by certain modi- 

 fying factors, while the total or latent potency remains relatively constant. 

 All types of differentiated cells, from this point of view, basically are totipotent; 

 that is, they possess the latent power to give origin to all the kinds of cells 

 and tissues of the particular animal species to which they belong. 



The specific potencies which denote the normal development of particular 

 organs undoubtedly have their respective, although often quite devious, con- 

 nections with the fertilized egg. However, one must concede the origin of 

 abnormal or acquired potency values due to the insinuation of special in- 

 ductive or modifying factors which disturb the expression of normal potency 

 value. For example, tumors and other abnormal growths and tissue distortions 

 may be examples of such special potencies induced by special conditions which 

 upset the mechanism controlling normal potency expression. 



2. Some Terms Used to Describe Different States of Potency 

 a. Totipotency and Harmonious Totipotency 



The word totipotent, as applied to embryonic development, was introduced 

 into embryological theory by Wilhelm Roux, and it refers to the power or 

 ability of an early blastomere or blastomeres of a particular animal species 

 to give origin to the many different types of cells and structures characteristic 

 of the individual species. Speculation concerning the meaning of totipotency 



