150 



AN INTRODUCTION TO MODERN GENETICS 



reacting tissue is itself able to produce some pattern properties. But we 

 are still largely ignorant of how extensive the pattern-forming pro- 

 perties of the competent tissues may be. As regards neural tissue, it 

 seems that the normal pattern is almost entirely dependent on the 

 action of the organizer, but in other organs the competent tissue may 

 be more important. 



There are already some indications of the relative importance of the 

 nucleus and cytoplasm in determining the pattern properties of organ- 

 izers and competent tissues. The formation of the pattern within the 



Fig, 73. Control of Pattern by the Organizer. — The two figures on the left 

 show an exchange-graft of ectoderm between a small light newt gastrula and a 

 large dark AxolotI gastrula. The figure on the right shows the AxolotI material 

 taking part harmoniously in the formation of the newt neural plate. 



(From Holtfreter.) 



organization centre seems to be connected with the original localization 

 of the organizer within the egg, and this process, as we have seen, is 

 mainly cytoplasmic or at least independent of the zygote nucleus. In 

 the unfolding of the pattern, both components appear to play a part. 

 Dalcq^ showed that eggs fertilized by sperm which had been injured by 

 X-rays or trypaflavine fail to gastrulate properly, probably because the 

 injury to the genetic constitution upsets the pattern of the organizer. 

 Similarly, Hamburger^ has described characteristic abnormalities of 

 development which must be due to incompatibility between the 

 chromosomes and cytoplasm. The same conclusion can be more cer- 

 tainly drawn from experiments on bastard merogony, that is, experi- 

 ments in which the egg nucleus was removed and the enucleated 



Dalcq and Simon 1932. 



^ Hamburger 1936. 



