THE MOSAIC STAGE OF DIFFERENTIATION 245 



of the eye is introduced by the fact that the organiser, in the form 

 of the primitive gut-roof, underhesthe neural plate, and it now plays 

 a part in the further determination of the eye-regions. The action of 

 the primitive gut-roof in this respect has been tested by grafting 

 portions of the eye-area without and with the primitive gut-roof.^ 

 It is found that the primitive gut-roof reinforces the eye-forming 

 potencies of the lateral portions of the eye-area, and, further, it 

 leads to the formation from median pieces of the eye-area of two 

 eyes, with optic stalks, and separated by a region of the floor of the 

 brain representing the optic chiasma ; whereas similar pieces with- 

 out gut-roof produce a single eye. The explanation of this "twin- 

 ning" effect of the gut-roof on the eye-rudiment is still to seek. 



The gut-roof or organiser further accelerates the processes 

 leading to progressive subdivision of the eye-region into chemo- 

 differentiated subregions. This is illustrated by experiments on 

 Rana escidenta. If at the neurula stage in embryos of this species 

 a rectangular piece is cut out from the neural fold region including 

 part of the eye-area (together with the underlying primitive 

 gut-roof), rotated through i8o°, and grafted back again so that 

 the original anterior edge of the piece is posterior, the rotated piece 

 undergoes self-differentiation. The result in the later embryo is a 

 reversal of the normal order of the structures of the brain : the di- 

 encephalon with the epiphysis lies behind the mesencephalon with 

 its optic lobes. Anteriorly and posteriorly, these structures which 

 have developed from the reversed piece are continuous with the 

 parts of the brain which have developed undisturbed. It may be 

 noticed, therefore, that the diencephalon, epiphysis, and optic lobes 

 were already determined as subregions in the neural fold field at the 

 time of operation,^ and there is evidence that the infundibulum is 

 also determined.^ 



But the most interesting feature of this experiment is concerned 

 with the eyes. It is frequently found that there is a small pair of 

 eye-cups in the normal position, and another pair farther back, 

 situated either in front of or behind the ear-vesicles. The explana- 

 tion of this result is that the anterior cut by which the rectangular 

 piece was separated from its surroundings passed through the 



^ Adelmann, 1930. 2 Spemann, 1912A. 



