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CONTINUITY OF LIFE 



the muscle cells, what would happen? This 

 complex moving of cells must occur at pre- 

 cisely the right time and to the right loca- 

 tion if the final organ is to form and func- 

 tion properly. Such an amazing series of 

 events certainly challenges man's ingenuity 

 to produce an explanation. 



Still another essential phase of develop- 

 ment must take place after these cells reach 

 their destination. The cells must differen- 

 tiate into specific kinds of cells in order to 

 produce the definitive organ. In general, 

 such differentiated cells can perform one 

 function and no other, and once formed 

 never revert to something else. There are 

 some cells, however, that do retain the 

 ability to become something else, for exam- 

 ple, the connective tissue cells that can fill 

 in the space resulting from a wound — the 

 scar tissue cells. Most cells, however, once 

 differentiated, perform no other function 

 than the one destined for them. 



One would expect the final differentia- 

 tion of the cells to occur as the last thing 

 in their long history. Consider, for example, 

 the central masses of cells that push out 

 into tiny limb buds which eventually form 

 the appendages of vertebrates. These cells 

 are all alike at first but they begin to dif- 

 ferentiate into parts that soon become dis- 

 cernible. The cells at the tips become flat- 

 tened and together rounded out into five 

 tiny bumps, the ultimate digits. Up to this 

 point the cells are all much alike, but they 

 soon become variously transformed into 

 cartilage in the center, witli muscle and 

 connective tissue of different kinds sur- 

 rounding it, and blood vessels and nerves 

 permeating the entire structure. Some cells 

 become associated with the ectoderm to 

 form the skin. When all of the cells have 

 completely differentiated into their final 

 structure the characteristic appendage 

 comes into being. 



Differentiation of the germ layers follows 

 a similar pattern in all animals, and the final 

 organs derived from them simulate rather 

 well what might be expected from the story 

 of evolution. That is to say, the layers that 



first formed ectoderm and endoderm might 

 be expected to give rise to the covering and 

 the lining of animals because those were 

 their functions in the first Metazoa. The 

 ectoderm, since it contacts the external 

 world, might also be expected to give rise 

 to the organ system that keeps it in touch 

 with its external environment, namely, the 

 nervous system and its associated organs. 

 The endoderm lined the gut of the first 

 Metazoa and it still performs that same 

 function in higher animals, giving rise as 

 well to such structures as the lungs, liver, 

 pancreas, and so forth. The last layer to 

 appear both in evolution and in embryol- 

 ogy is the mesoderm, which might logically 

 produce all of the remaining organs such as 

 muscles, blood, vessels, skeleton, and so on. 

 Although, in general, the three germ layers 

 are destined to produce specific organs, 

 there are rather interesting deviations from 

 the set pattern, some of which lend them- 

 selves to experimentation. 



FACTORS INFLUENCING 

 DEVELOPMENT 



Development does not always follow the 

 age-old pathways in the production of em- 

 bryos, as attested by the occasional appear- 

 ance of malformed offspring among all 

 groups of animals. This means that some- 

 thing has gone wrong along the way in the 

 formation of the various systems. Perhaps 

 some chemical or physical factor did not 

 function quite as it usually does. The fact 

 that such things happen opens the way for 

 experimentation by purposely interrupting 

 the normal course of events. This fertile 

 field of investigation, called experimental 

 embryology, has opened new vistas in our 

 understanding of normal development. It 

 is interesting to note that several Nobel 

 Prizes in medicine have been awarded to 

 experimental embryologists whose efforts 

 have been concentrated primarily on the 

 embryos of lower animals such as the frog, 

 a fact that is often difficult for the layman 

 to understand. It simply means that in order 



