4 THE ROYAL SOCIETY OF CANADA 



to the internal limiting membrane. Note once more the intimate 

 relation of one of these primitive axons to the pole of the nucleus at X. 

 It is unfortunate that the axons do not stain well during the early 

 stages, which indeed represent the critical phases of their histogenesis. 

 On this account they are never displayed to advantage in micro- 

 photographs, and therefore the problem of their origin would be best 

 discussed in the laboratory over the microscopic field. Fortunately, 

 however, both Bernard and the author have been able to demonstrate 

 that the neuroblast-nuclei which later form the external nuclear layer 

 of the retina extrude their achromatic contents into the bases of the 

 developing rods and cones, while the writer (6) has also described this 

 discharge of material by neuroblast-nuclei in other parts of the develop- 

 ing nervous system. There is thus a growing belief in the fact that 

 the nuclei, of embryonic tissues at least, are great centres of metabolic 

 activity, the latter being manifested by the discharge at regular 

 intervals of a peculiar material which in the nascent condition is 

 practically achromatic in its reaction towards staining agents. The 

 production of this material has been previously demonstrated by the 

 writer (3) in embryonic striated muscle, and by Dr. R. J. Gladstone 

 and the writer (2) in the developing blastoderm. 



From this viewpoint the causation of the overturning of the 

 neuroblast-nuclei that give rise to the ganglionic layer of the retina is 

 probably as follows : 



1. The primitive axis cylinders of the amphibian optic 

 nerve are partly formed by the discharge of achromatic material 

 from the outer poles of these nuclei. 



2. So far as the author can at present determine the axon is 

 always connected with the outer nuclear pole. 



3. These axons become in due course directed towards the 

 point of exit of the optic nerve from the retina. 



4. They exert traction at a very early stage upon the nuclear 

 poles from which they have emerged, the result being that these 

 poles, which might appropriately be termed the "axon (6)," are 

 rotated through an angle of approximately 90° and become m 

 every case directed towards the optic disc. 



The question that next arises is. Why do the neuroblast-nuclei 

 that give rise to the external and internal nuclear layers of the retina 

 exhibit no evidences of this overturning movement. The answer to 

 this is not difficult. These nuclei discharge achromatic material 

 from both their outer and inner poles so that a condition of equilibrium 

 results. The description of the phenomena associated with these outer 



