In speaking of growth or development those who do not give close attention to the problems involved in this phenomenon usually emphasize primarily the increase in size or weight which familiarly characterize it. The growth of the organism as a whole, particularly at an early period, is accompanied by another feature, that of differentiation. The changes in form and weight do not consist merely of additions to the tissue elements already laid down, but frequently also include those alterations whereby new types of structure are added to or evolved out of the primitive ones originally present.
The striking experiments of Dr. Gudernatsch1 of the Department of Anatomy at Cornell University Medical College in New York lend a peculiar emphasis to the fact that in development we may deal with two entirely separate factors, one of growth and one of differentiation. This was brought out by feeding to developing tadpoles
THE THYROID AND THYMUS IN RELATION TO DEVELOPMENT. JAMA. 1914;LXIII(1):34–35. doi:10.1001/jama.1914.02570010036013
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