THEORIES REGARDING ORGANIZATION 



41 



there are different substances in the animal and vegetal halves of the egg. 

 These substances are responsible for the difference in the way the two halves 

 develop. But the animal half contains some of the substances of the vegetal 

 half, and the vegetal half contains some of the substances of the animal half. 

 Thus two gradients are postulated: one of animal substances and one of 

 vegetal substances (Fig. 19). When these two gradients balance each other, 

 normal development ensues. If one is depressed by a chemical treatment 

 the other gradient takes over. When the egg is treated with potassium 

 sulfocyanide, for example, the vegetal substances are poisoned and only the 

 animal substances are left to act in development. As a result, the egg forms a 

 permanent blastula. On the other hand, if the egg is treated with lithium 

 chloride the animal substances are inhibited, the ectoderm fails to develop, 

 and the vegetal substances act to produce endoderm. 



As we shall see when we go on to the analysis of later development, a 

 compromise theory is reasonable. Quantitative differences have been meas- 

 ured within the developing egg, but there are also qualitatively different 

 substances present in different regions. The differentiation of the parts of 

 the embryo would then be determined both by the kind and by the amount 

 of the substance present. 



In any case, whatever the fundamental organization of the egg, a 

 stimulus is needed to convert this organization into the structures of the 

 adult. The unfertilized egg dies in a matter of hours unless a sperm pene- 

 trates it or some other stimulus is applied. After stimulation the chemical 

 substances in the egg are able to react with each other to produce the typical 

 changes which occur during development. What is the nature of the stim- 



Fig. 19. A two-factor hypothesis of the 

 organization of the egg. Two factors, A, an 

 animal factor, and B, a vegetal factor, are 

 present in the egg. Factor A converts regions 

 of the egg into ectoderm, B into mesoderm 

 and endoderm. A is highest at animal pole 

 and grades off toward vegetal pole, A S) 

 A 1 . . . A x . ■ B is highest at the vegetal pole 

 and grades off toward the animal pole, B 8 , 

 B- . . . B v 



GRADIENT OF ANIMAL FACTORS 

 A 



GRADIENT OF VEGETAL FACTORS 



