Genes: Nature and Mode of Action - 535 



1st TAIL i st VERTEBRA 1st EAR 



1st NEURAL 

 TUBE 



nd DORSAL 

 LIP 



1st DORSAL 

 LIP 



2nd TAIL 



2nd 

 VERTEBRA 



NEURAL 

 TUBE 



2nd EAR 



YOUNG EMBRYO 



OLDER EMBRYO 



Fig. 27-8. Left, an embryo at the beginning of gastrulation. An organizer from another embryo of the same 

 stage has been transplanted, in the position shown. Right, the same embryo at a later stage. 



an entirely foreign region of the embryo. 

 Thus the dorsal lip of the blastopore serves as 

 the primary organizer in the vertebrate em- 

 bryo. In fact the organizing capacity of the 

 lip region can be traced back before gastrula- 

 tion. If the corresponding part of a fertilized 

 egg is removed, such an egg never develops 

 into anything but a ball of cells; whereas if 

 other small parts of the egg are taken away, 

 the egg still gives rise to a normal or approxi- 

 mately normal embryo. 



After gastrulation, other parts of the em- 

 bryo, especially those lying near the primary 

 organizer, gradually become determined. 

 Soon a bit of ectoderm, transplanted from 

 the dorsal surface quite some distance an- 

 terior to the blastopore, will develop into 

 nerve cord regardless of its new location. 

 Moreover, this ectoderm acts as a secondary 

 organizer in that it determines not only its 

 own development, but also that of nearby 

 parts. And in like manner, other parts of the 

 embryo, once they have been determined and 

 launched upon some special line of inde- 

 pendent development, may act as organizers 

 of such parts as may still remain in a non- 

 determined state. 



Precisely how an organizer "organizes" is 

 quite unknown. Perhaps it produces special 

 substrates that, transmitted to neighboring 



cells, change the enzyme induction pattern 

 of these cells. The dorsal lip tissue will in- 

 duce the formation of a neural tube even if 

 the lip tissue is killed before it is introduced 

 into the host embryo. In fact, supernumerary 

 neural tubes have been induced by a variety 

 of chemical and physical agencies, some quite 

 foreign to the normal embryonic environ- 

 ment. At present, all that can be said is that 

 the organizer introduces some kind of differ- 

 ential into the environment of the genes of 

 neighboring cells, and this differential in- 

 duces a change in the subsequent behavior 

 of these genes. Certainly the different parts 

 of an embryo have ample opportunity for 

 affecting one another: mechanically, as a 

 result of mutual pressure; chemically, as a 

 result of differences in the rates and qualities 

 of metabolism; electrically, by way of bio- 

 electric phenomena; and so forth. Moreover, 

 other factors that are still obscure may play 

 upon the genes in the different parts of the 

 embryo, determining the course of their per- 

 formance within the sphere of their po- 

 tentiality. 



Probably every gene is present in all the 

 diploid cells of the organism, and therefore 

 every gene may have some effect upon the 

 metabolism of every cell. More or less con- 

 stantly the genes must continue to use up 



