1 82 READINGS IN BIOLOGICAL SCIENCE 



organs. It has been possible to make maps of certain eggs and in many cases 

 to point out, with great accuracy, the exact material that will give rise to 

 certain organs. We know, for example, just what materials will go into the 

 making of the brain, what material will form the digestive canal, the 

 muscles, bones, and so on. 



Embryonic development is, in the main, a continuous procedure, once 

 fertilization has occurred. Two outstanding processes are always in evi- 

 dence; cell multiplication and cell differentiation. Thus, starting with the 

 fertilized egg, the new organism grows in size and its parts become dif- 

 ferentiated and specialized. The fertilized egg, which is a single cell, by 

 continued division forms first an aggregate of many cells. Then the many- 

 celled aggregate begins to undergo differentiation into special layers of 

 cells, in particular, an outer layer, called the ectoderm, an inner layer or 

 entoderm, and a middle layer, or mesoderm. Each of these three layers 

 has a definite significance, or fate, in normal development, and, as stated 

 above, maps showing the presumptive fate of different regions can be 

 made. For example, the ectoderm in one region gives rise to the brain, 

 whereas in other regions it goes into the formation of the epidermis of 

 the skin. The heart, blood vessels, and all bone and muscle come from the 

 mesoderm, and so on. 



With an extensive knowledge of normal embryology, investigators 

 naturally began to try experiments with the developing egg and embryo, 

 and hence has grown up the field of experimental embryology. Experi- 

 ments can best be performed on eggs and embryos which develop outside 

 the body of the mother. Among the most favorable and most commonly 

 used are embryos of the amphibia, such as the frogs and newts. Eggs of 

 these animals are shed by the female into the water, where fertilization 

 takes place, and the embryo develops in its aqueous environment, shielded 

 only by certain protective memibranes. Hence, we have a readily available 

 supply of embryos. By removal of the outer protective membranes, the 

 experimenter can easily secure naked embryos, and, by the use of suitable 

 small instruments and appropriate microscopes, can perform many opera- 

 tions in these embryos, such as extirpating pieces of an embryo, and trans- 

 planting cells from one location to another. 



What would happen, for example, if in an early stage of development 

 a group of cells, which normally would form a part of the brain, was re- 

 moved and cells transplanted in their place which normally would form 

 epidermis of the skin? In other words, suppose that presumptive brain 

 cells be replaced by presumptive epidermal cells. This and many similar 

 experiments, have been performed during recent years by experimental 

 embryologists in this country and abroad. From these experiments we 

 learn that cells which normally would form the epidermis of the skin in 

 the belly region of the embryo, if transplanted to the proper site at the 

 proper time, will develop into nerve cells of the brain. Conversely, cells 



