10 



EMBRYOLOGY 



Fig. 5. We know which of the cells 

 in the developing egg give rise to 

 the brain and which give rise to the 

 skin or, more properly, the epider- 

 mis. These two regions are then cut 

 out and interchanged, as shown by 

 the arrows connecting the two eggs. 

 The light-colored egg, which has 

 had some of its future brain re- 

 placed by future epidermis, develops 

 into an embryo with a normal brain. 

 The dark egg, with future brain in 

 the region of the future epidermis, 

 develops into an embryo with a nor- 

 mal epidermis. Since parts of the 

 early egg can be interchanged, what 

 factors are responsible for the de- 

 velopment of brain and epidermis? 



other tissues, and, conversely, cells destined to become the early nervous 

 system can form skin. 



The factors which determine the ultimate fate of cells must almost cer- 

 tainly be reduced to chemical factors of some sort, and if we had these sub- 

 stances at our disposal we would be able to control differentiation of groups 

 of cells. The control of differentiation is extremely important in a number 

 of processes in the adult, such as the healing of wounds and the replacement 

 of lost tissue. As a matter of fact, if one lets his imagination go, there 

 is no reason why he cannot foresee the day when we shall be able to cultivate 

 large numbers of tissue cells, just as bacteria and yeast are cultivated now. 

 Then, given the chemical factors, we shall be able to cause these cells to 

 develop into almost any type of tissue we want. Further, given the proper 

 specificity relationships — such as in blood groups — we may use them in trans- 

 plantation for the healing of wounds and injuries. Such a program, of course, 

 requires a strong flight of the imagination, for what we should like to do 

 is a far cry from what we can actually do now. 



In many animals embryonic processes may occur even in the adult. The 

 problem of regeneration is essentially a problem of embryology. A planarian 

 worm with its head cut off will regenerate a new head with nerve ganglia 

 and all the other structures originally present in the intact head. Salamanders 

 will regenerate new limbs or tails when they lose either of these structures. 

 Some animals may be cut into a dozen or more pieces and each piece will 



