THE DEVELOPMENT OF THE INDIVIDUAL 539 



When an undifferentiated limb bud, which is just a thin patch of 

 mesoderm, is grafted onto a position midway between the front and hind 

 limb positions on a salamander embryo, it will usually develop into a 

 nearly perfect limb. It will acquire nerves from the adjacent segments 

 of the spinal cord, but will be devoid of adaptive function because it 

 does not obtain the proper reflex connections in the spinal cord. Even 

 a small part of a limb bud is capable of producing a complete limb as 

 can be shown by grafting the anterior half of the bud to one place and 

 the posterior half to another location. In this experiment two com- 

 plete limbs develop from one limb bud. Evidently, the cells in the half 

 bud are able to restore and reorganize the missing parts of the other 

 half bud — in other words, the cells have become bud-forming cells and 

 are capable of forming any part of the bud. We call this a restitution 

 system. 



Salamanders have cells scattered through their bodies which retain 

 most of their primitive characteristics and are capable of regenerating 

 limbs even in the adult stages. Even in man there are connective tissue 

 cells which form the scar tissue that closes wounds. The skin also re- 

 generates rapidly. One theory of cancer holds that these cells, which 

 are so important in cases of injury, get out of control and begin divid- 

 ing wildly, forming parasitic tissue. 



In summary, it is clear that since all the cells of a developing embryo 

 have the same genes, some mechanism must appear that will cause these 

 cells to develop into different tissues and organs. This is believed to 

 be due to some chemical substance or substances that become concen- 

 trated in the dorsal lip of the blastopore. Before the action of this 

 structure during gastrulation, all of the cells of the embryo are equally 

 potent in producing any part of the embryo, but after the action of this 

 organizer the cells have become so modified that they can produce only 

 particular organs. Although embryonic differentiation is due to chemi- 

 cal changes in the cytoplasm, experiments have shown that the genes in 

 the chromosomes play an important part in determining the type and 

 qualitv of various structures as they form. 



REVIEW QUESTIONS 



1. Describe the main types of cleavage in the chordates. 



2. Describe the different methods of formation of the blastula and the gastrula. 



3. How is the mesoderm derived and the coelom formed in the different 

 chordates ? 



4. What systems arise chiefly from the endoderm, mesoderm, ectoderm? 



5. Describe the embryonic development of the nervous system. 



6. Describe the embryonic development of the respiratory system. 



7. Name the outgrowths of the skin and give the germ layers from which each 

 is derived. 



