Development 63 



This picture of development is supported by a vast array of data 

 from experimental embryology. Interactions of cells may be seen in 

 cultures of microorganisms in which density of the culture mav affect 

 rate of growth ( or determine whether growth is possible at all ) . The 

 literature on induction (by contact or at a distance) and organizers 

 testifies to the potency of effects of cellular environment and to the 

 complexitv of the systems that have evolved. None of these data, 

 however, demonstrates that the genetic information takes merelv a 

 passive role in development. That mitosis does not parcel out por- 

 tions of genetic information to the proper parts of the developing 

 organism seems certain. Experiments such as those mentioned above 

 on Flatijcnemis have demonstrated that in most organisms, at least, 

 the earliest cleavage cells are totipotent, retaining the information 

 necessary for the development of the entire organism. In addition, 

 cells taken from various parts of the body and examined micro- 

 scopically do not seem to be deficient in their chromosome content, 

 as would be expected if gross partitioning took place. In those in- 

 sects that have polytene chromosomes with distinct banding in more 

 than one body tissue, it has been reported that the banding does not 

 change from tissue to tissue. 



The study of these giant chromosomes has provided other critical 

 data for the interpretation of development. During the course of 

 ontogeny certain bands become enlarged tremendously and are 

 known as pujfs. This puffing process is reversible. At the same stage 

 in other tissues, different bands are in the puff condition. Clearly 

 there is differential behavior of the chromosomal material, but the 

 mechanisms controlling this behavior are unknown. It is assumed 

 that the presence of puffs is an expression of gene action. The num- 

 ber of puffs that may develop on a chromosome is much lower than 

 the number of bands; therefore the activity of other parts of the 

 genetic code must be "invisible." 



If the genetic material participates more actively in differentia- 

 tion than outlined above, it must do so in a very subtle manner. In- 

 teresting recent experiments of Briggs and King have shown that the 

 nuclei in the cells of some embryonic tadpoles are in some way 

 altered in the course of development. When the nucleus of a frog 

 egg was removed and replaced with one from a frog blastula, normal 

 development ensued. When it was replaced with a nucleus removed 

 from a gastrula or neurula, deformed embryos resulted in which the 

 only normal tissues were those derived from the germ layer from 

 which the donated nucleus was taken. While this does not necessarily 

 indicate that the genetic information has been altered in the course 

 of development, it certainly does not militate against that hypothesis. 



