258 CHARLES R. STOCKARD 



intermixed with those cells destined to form other retinal layers, 

 ^nd so produce an eye with the pigment cells scattered irregu- 

 larly through the retina. Spemann's recent experiments on cut- 

 ting out and reversing certain areas of the open medullary plate 

 indicate that the tapetum cells are fully localized and separate 

 from the other retinal cells in certain amphibian embryos. The 

 embryos in my experiments had not differentiated the retinal 

 layers sufficiently far to determine with certainty whether there 

 was a persistent disarrangement of the cells, yet the general 

 appearance of the eyes seemed perfectly normal. 



2. The median region of the anterior part of the 7nedullary plate 

 cut out, reversed and transplanted in the medullary plate 



This experiment is similar to 'fehose performed b}^ Spemann 

 on several amphibian embryos. Spemann found that pieces of 

 the medullary plate when cut out and turned around continued 

 to develop with their original orientation undisturbed, thus indi- 

 cating the early prelocalization of certain future parts of the 

 brain and eyes. When the operation chanced to cut the eye 

 anlage so that part of the future eye material was anterior to 

 the cut and remained in position, while part was contained in 

 the cut-out piece which was then turned around and transplanted, 

 carrying the future eye cells to a more posterior position, two 

 eye regions developed on each side. One arose from the ante- 

 rior undisturbed cells and the other from the transplanted pos- 

 terior cells. 



The reversed pieces in the present experiment were not long 

 enough to carry the eye back to a distant posterior position, 

 and the cut extended so far foreward that the eye anlage was 

 not divided transversely as in Spemann's operations. The oper- 

 ations were done chiefly to test whether the eye anlage in Ambly- 

 stoma was well localized and would develop after such reversal 

 of tissues. 



Eight embryos were studied after having had antero-median 

 pieces of the medullary plate cut out, reversed and transplanted. 

 Seven of the eight developed both eyes, many of which showed 



