POTENCY IN RELATION TO DIFFERENTIATION 377 



of a single blastomere received encouragement from the discovery by Hans 

 Driesch, in 1891, that an isolated blastomere of the tWo- or four-cell stage 

 of the cleaving, sea-urchin's egg could give origin to a "perfect larva." Driesch 

 described this condition as constituting an equipotential state, while Roux 

 referred to it as a totipotential condition. As the word totipotential seems 

 more fitting and better suited to describe the condition than the word equi- 

 potential, which simply means equal potency, the word totipotency is used 

 herein. The word omnipotent is sometimes used to describe the totipotent 

 condition; as it has connotations of supreme power, it will not be used. 



The totipotent state is a concept which may be considered in different 

 ways. In many instances it has been used as described above, namely, as a 

 potency condition that has within it the ability to produce a perfect embryo 

 or individual. The word also has been used, however, to describe a condition 

 which is capable of giving origin to all or nearly all the cells and tissues of 

 the body in a haphazard way but which are not necessarily organized to produce 

 a normally formed body of the particular species. Therefore, as a basis for 

 clear thinking, it is well to define two kinds of totipotency, namely, totipotency 

 and harmonious totipotency. The former term is used to describe the ability 

 of a cell or cell group to give origin to all or nearly all the different cells and 

 tissues of the particular species to which it belongs, but it is lacking in the 

 ability to organize them into an harmonious organism. Harmonious totipo- 

 tency, on the other hand, is used to denote a condition which has the above 

 ability to produce the various types of tissues of the species, but possesses, 

 in addition, the power to develop a perfectly organized body. 



The fertilized egg or the naturally parthenogenetic egg constitutes an har- 

 monious totipotential system. This condition is true also of isolated blasto- 

 meres of the two- or four-blastomere stage of the sea-urchin development, 

 as mentioned above, of the two-cell state of Amphioxus, or of the first two- 

 blastomere stage of the frog's egg when the first cleavage plane bisects the 

 gray crescent. However, in the eight-cell stage in these forms, potency be- 

 comes more limited in the respective cells of the embryo. Restriction of 

 potency, therefore, is indicated by a restriction of power to develop into 

 a variety of cells and tissues, and potency restriction is a characteristic of 

 cleavage and the blastulative process (figs. 61; 163A; 163B). When a stage 

 is reached in which the cells of a particular area are limited in potency value 

 to the expression of one type of cell or tissue, the condition is spoken of as 

 one of unipotency. A pluripotent state, on the other hand, is a condition in 

 which the potency is not so limited, and two or more types of tissues may 

 be derived from the cell or cells. 



b. Determination and Potency Limitation 



The limitation or restriction of potency, therefore, may form a part of the 

 process of differentiation; as such, it is a characteristic feature of embryonic 



