386 THE BLASTULA IN RELATION TO INNATE CONDITIONS 



development of the late blastula, however, the posterior quadrant normally 

 acquires a dominant condition together with a faculty for producing har- 

 monious totipotency. The other totipotent areas then become suppressed. 

 These basic conditions, therefore, serve to explain the experiments by Stockard 

 ('21 ) referred to above, where two organization centers tend to become domi- 

 nant as a result of isolating physiological conditions which tend to interfere 

 with the processes working toward the development of but one center of organi- 

 zation. This probable explanation of the twinning conditions in the teleost 

 blastoderm suggests strongly that the separation and isolation of separate 

 organization centers is a fundamental condition necessary for the production 

 of monozygotic or true twinning. 



It becomes apparent, therefore, that, in the development of the trout blas- 

 toderm (blastula), the development of an area which possesses a dominant 

 organization center is an important aspect of blastulation. In other blastulae, 

 the seat or area of the organization center apparently is established at an 

 earlier period, as, for example, the gray crescent in the amphibian egg which 

 appears to be associated with the organization center during the late blastula 

 state. Similarly, in the teleost fish, Carassins, totipotency appears to be limited 

 to one part of the early blastula (Tung and Tung, '43). 



It also follows from the analysis in the foregoing paragraphs that in the 

 production of polyembryony in the armadillo or of spontaneous twinning in 

 forms, such as the trout (Salmo), a generalized totipotency throughout the 

 early blastoderm is a prerequisite condition. When a single dominant area 

 once assumes totipotency, it tends to suppress and control the surrounding 

 areas, probably because it succeeds in "monopolizing" certain, substrate, 

 "food" substances (Dalcq, '49). 



£. Importance of the Organization Center of the Late Blastula 



It is also evident that one of the main functions of cleavage and blastulation 

 is the formation of a physiological, or organization, center which must be 

 present to dominate and direct the course of development during the next 

 stage of development. Consequently, the elaboration of a blastocoel with the 

 various, presumptive, organ-forming areas properly oriented in relation to 

 it is not enough. A definite physiological condition entrenched within the 

 so-called organization center must be present to arouse and direct the move- 

 ment of the major, organ-forming areas during gastrulation. 



