282 PATTERNS AND PROBLEMS OF DEVELOPMENT 



The many examples of embryonic developmental fields given by Huxley 

 and De Beer (1934) and by Weiss (1939) make it unnecessary to do more 

 than call attention to a few points of interest here. 



THE EYE FIELD OF AMPHIBIAN AND CHICK 



His earlier experiments led Spemann to believe that the optic primordia, 

 even those of cyclopian eyes, are determined and localized in the neural 

 plate as a mosaic of independently developing parts.' Later experiment 

 showed the incorrectness of this view (Spemann und Bautzmann, 1927). 

 The optic primordium was regarded by Stockard (19 13 and earlier papers) 

 as first median, spreading laterally and giving rise to two growth centers. 

 According to this view, cyclopia in the amphibian and probably in other 

 vertebrates is fundamentally different from cyclopia in a planarian and 

 from the approximation to, and development in, the median plane of the 

 anal arms of the sea-urchin larva with differential inhibition. There are 

 no grounds for believing that the primordium of the planarian eyespots 

 is originally median, and the primordium of the sea-urchin arms certainly 

 is not. The starfish coeloms become median and single with differential 

 inhibition (p. 219), but no reason appears for postulating an originally 

 median primordium. Moreover, the suckers of the anuran head and the 

 olfactory pits show the same graded approximation to the median plane, 

 single median development, and, with extreme inhibition, absence. Here, 

 also, there is no ground for postulating an originally median primordium. 



More recent investigation shows the existence in Amhlystoma embryos 

 of an eye field including anterior, median, and lateral regions of the neural 

 plate back to a certain level.'' Pieces from different parts of this region 

 transplanted to the belly wall give rise to eyes. According to Adelmann 

 in transplantation of pieces without mesentodermal substrate eye develop- 

 ment is much more frequent in pieces from the median region (70.8 per 

 cent of transplants) than in lateral pieces (ii.i per cent), that is, eye 

 potency is apparently much higher in the median than in the lateral 

 regions of the field. In transplants with the underlying tissue which in- 

 duced development of the neural plate 72.6 per cent of the median and 

 54.4 per cent of lateral pieces develop eyes; that is, the underlying 

 mesentoderm increases eye frequency in lateral but not in median pieces. 

 Moreover, of the median transplants with substrate, 70.8 per cent give 



3 Spemann, iqokz, 1903a, 1904, 1912a, h; see also Fischel, 1921, and H. Petersen, 1924. 

 '• Adelmann, 1929a, b, 1930, 1934, with references to earlier work; also Leplat, 1919; Man- 

 chot, 1929. 



