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V 



Embryology 



WE have come a long way from the viewpoint that the tgg and the 

 sperm contain the offspring in miniature. Our modern microscopes 

 have shown the absurdity of this notion. The essential parts of the gametes 

 are the chromosomes with their genes. The latter are chemical substances 

 which in some as yet mysterious way are concerned with the development 

 of all the characteristics of the adult. 



Embryology is concerned with the development of the fertilized t^g 

 into a mature embryo or possibly farther. Cell division is mitosis and the 

 development of daughter cells into one kind of tissue or another is dif- 

 ferentiation. The fact that two daughter cells, presumably with the same 

 genes will develop into dissimilar tissues is startling and poses a problem 

 which has not as yet been solved. That these cells, tissues, and organs dif- 

 ferentiate in the right place and develop their proper function at the right 

 time is an even greater cause for wonderment. 



There are many theories to account for these mysteries but it must be 

 remembered that a theory does not become a law simply by constant repe- 

 tition. The mysteries of the cell require much work before we can gain 

 a glimpse of the truth. 



An attractive theory for which proof seems to be lacking is that the 

 genes in the nuclei of daughter cells are 7iot similar to each other but that 

 some mechanism is operative which changes the chemical structure of 

 genes to suit the future use to which the cell is to be put. This does not 

 imply the operation of an outside force but may be something hereditary 

 within the genes themselves or it may be environmental in nature. 



EMBRYOLOGY * 



LESLIE BRAINERD AREY 



Originally 'embryology' was a term restricted to the events of prenatal 

 development. Only gradually was it realized that developmental proc- 

 esses continue long after birth. One branch of embryology traces the 



* Reprinted from Developmental Anatorny by Leslie Brainerd Arey with the per- 

 mission of W. B. Saunders Company and the author. Copyright 1940 by the W. B, 

 Saunders Company. 



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