354 WILLIAM H. COLE 



INTRODUCTION 



The fact of dependent differentiation has been observ^ed in the 

 development of several animals. It has been demonstrated that 

 certain cells of the early embryos will differentiate into the anlage 

 of an organ, providing certain other cells are present to originate 

 the stimulus for such a change. In some cases by appropriate 

 operations, a third group of cells from a different region of the 

 embryo may replace the first group, and will show the same kind 

 of differentiation. Such 'totipotence' of embryonic cells may 

 persist to a certain degree in the adult state of lower invertebrates, 

 as shown, for example, by the remarkable regenerative power of 

 coelenterates and worms. But in the higher vertebrates totipo- 

 tence is lost completely, though at different stages of develop- 

 ment in different animals. 



During the past twenty years the organogeny of the vertebrate 

 eye has been investigated extensively to discover any correlation 

 that may exist between the development of one part and that of 

 another. In these studies certain parts have been injured, re- 

 moved, transplanted, or replaced by strange parts, in attempts to 

 solve the problem. The researches of Spemann ('01, '03, '07, '12), 

 Lewis ('04, '05), King ('05), Le Cron ('07), Stockard ('10), and 

 Fessler ('20) are the most noteworthy. In general it has been 

 concluded that a certain amount of correlation between the de- 

 velopment of the parts of the eye exists, but that also some of the 

 parts have become, or are becoming, self-differentiating. For 

 example, the formation of the lens is stimulated and aided by the 

 optic cup, although under certain conditions it may develop with- 

 out the influence of the optic cup (Stockard, '10). A second 

 example is furnished by the appearance of transparency in the 

 ectoderm over the eye. In normal development the ectoderm 

 over the eye loses its pigment, is thinned, and becomes transpar- 

 ent. If ectoderm from a strange region of the embryo replaces 

 the normal ectoderm, the former becomes similarly transparent. 

 If, however, the eye is removed, no transparency results. These 

 discoveries have enlivened the age-old discussion concerning the 

 relation between the form and function of an organ. Modern 

 biologists may be divided into two groups, according to their 



