DEVELOPMENT OF THE INDIVIDUAL 



577 



blas+emci 



Fig. 23-15. An amputated limb of a larval salamander will regenerate completely. Why does a mammalian 



appendage fail to regenerate? 



if indeed they have? Some interesting ex- 

 periments have thrown hght on these prob- 

 lems. 



When a hmb of a larval salamander is 

 amputated, the epidermis quickly grows 

 over the cut surface while a concomitant 

 series of changes go on underneath result- 

 ing eventually in a new limb (Fig. 23-15). 

 The cells of the cut muscle, dermis, and 

 bone change in appearance. They dediffer- 

 entiate, that is, lose most of their specializa- 

 tion, and eventually form a small region of 

 special cells called a blastema. Once the 

 blastema is formed further dedifferentiation 

 ceases. The cell mass then begins to re- 

 differentiate into the various parts of the 

 limb, that is, into bone, muscle, skin, ves- 

 sels, and nerves. Thus the lost part is com- 

 pletely formed and becomes functional. 



To go back to our earlier question, why 

 does this fail to occur in the closely related 

 frogs and other vertebrates, man for ex- 

 ample? Further experimentation tells us 

 something more about this. If the cvit limb 

 of a frog is placed in saline (salt solution) 

 it will regenerate completely. Why? Ap- 

 parently, the saline prevents the normal 

 closing over of the wound and brings about 

 the formation of a blastema which normally 

 does not appear. Once the blastema is 

 formed, regeneration proceeds just as it 

 does in salamanders. It appears that the 

 healing skin of the amputated frog limb 

 blocks further regeneration, because if the 

 skin of the salamander appendage is made 

 to cover the cut end, regeneration also fails, 

 whereas if just the epidermis, which nor- 



mally covers the opening, appears, regen- 

 eration proceeds in a normal fashion. It 

 seems, therefore, that the ability to regen- 

 erate appendages is not lost, but merely 

 blocked. What, then, about the regenera- 

 tion of appendages or other parts in mam- 

 mals? That must await future research. 



In all of these experiments we see the 

 direct bearing of tlie environment on devel- 

 opment. Animals possess certain genetic 

 potentialities which will be realized in a 

 specified environment. If this environment 

 changes in any way, corresponding altera- 

 tions occur in the developing embryo. The 

 two factors of hereditv and environment are 

 both important in development. We shall 

 have more to say about them in the next 

 chapter. 



So far we have discussed the basic prob- 

 lems involved in development. Intimately 

 linked with development is growth, and, in 

 fact, the two go hand in hand. The embryo 

 grows continually while it is differentiating. 

 In the light of what we know about devel- 

 opment, let us look at this problem of 

 growth. 



GROWTH 



By the end of the eighth week of human 

 life the embryo has increased to 2 million 

 times its original mass and during the next 

 eight lunar months it will increase another 

 4,000 times. This represents a tremendous 

 accumulation of protoplasm which we call 

 growth. What are some of the facts con- 

 cerning this important vital phenomenon? 



