[CAMERON] NEUROBLAST-NUCLEI 3 



but those nearest to the internal limiting membrane become tilted 

 over more and more until their long axes ultimately lie parallel to 

 the internal limiting membrane. That is to say each of these nuclei 

 becomes rotated through an angle of approximately 90°. The writer 

 can find no record of this alteration in orientation having been pre- 

 viously observed. However, it can be detected without difficulty in 

 the frog-embryo nineteen days after fertilization. It is interesting 

 to watch these nuclei becoming gradually tilted over more and more 

 by some unseen force during the next few days, which certainly 

 exhibit great strides in the development of the amphibian retina. 

 This rapid advance is, of course, due to the fact that in the very early 

 stages of its career the frog-embryo becomes a free swimmer and 

 therefore the development of its optical apparatus has the full light 

 of day as nature's stimulus to accelerate histogenesis. It is therefore 

 remarkable to note how much progress has been accomplished even 

 by the twenty-first day (see Fig. 4), though this particular embryo 

 proved to be a rather precocious example. The latter Fig. afforded 

 the clue as to the direction in which the neuroblast-nuclei were tilted, 

 for it was found that the overturned nuclear poles became directed 

 consistently towards the point of exit of the optic nerve fibres at 

 the optic disc (Fig. 4). 



If these ganglionic nuclei be closely studied by means of the 

 highest powers of the microscope from the nineteenth day onwards 

 it will be ascertained that a portion at any rate of each primitive 

 axis-cylinder is discharged from its associated nucleus in the form of a 

 material which in the nascent condition is very resistant to staining 

 agents. In many cases this discharge is so active that the nuclear 

 pole is drawn out as a fine point into the commencement of the axis- 

 cylinder (see the nuclei marked with an X in Fig. 3). One of the most 

 remarkable facts- regarding these amphibian neuroblast-nuclei is 

 that in the early stages they are practically devoid of a cytoplasmic 

 investment (Fig. 3). Even at the thirty-fifth day (see Fig. 5) the 

 perinuclear material is still of the scantiest. Indeed, it is only towards 

 the end of development that the cytoplasm of the ganglionic cells can 

 be detected as a distinctive investment. 



This comparative nakedness of the retinal neuroblast-nuclei 

 during the early developmental stages compels one to look for another 

 source of the optic nerve axons since these make their appearance 

 as early as the nineteenth day. For example, the narrow band of 

 faintly stained material extending to the left from the nuclei marked X 

 in Fig. 3 represents the earliest rudiment of the retinal layer of optic 

 nerve fibres, which, as the Fig. shows, lie in t)ieir usual position next 



