2 THE ROYAL SOCIETY OF CANADA 



useful bibliography will be found in a recent paper by Detwiler (4) 

 on this subject. The most elaborate study of the embryonic retina 

 within recent years is that by Bernard.(5) I wish that space would 

 permit me to do full justice to the work done by this observer. We 

 both chanced to be working on the embryology of the retina simul- 

 taneously and unknown to each other. It was, however, remarkable 

 to note how closely our results were in agreement. Bernard's work 

 will, therefore, be found referred to in the course of this paper. 



If the inner wall of the optic cup be examined by the higher 

 powers of the microscope it will be found to exhibit the same structural 

 arrangement as the wall of the neural tube. This is just as one would 

 expect seeing that the cup is a derivative of the latter. Thus, one 

 will observe next to the external limiting membrane^ the usual layer 

 of germinal nuclei in active karyokinesis. Now, if the direction of 

 separation of the daughter-nuclei be studieS it will be ascertained 

 that this is usually the same, namely, parallel to the external limitmg 

 membrane (see Fig. 1). Further, it should be noted that the long 

 axes of the daughter-nuclei lie always at right angles to the external 

 and internal limiting membranes, as shown in Fig. 1 and Fig. 2. 



If the relationship of the germinal nuclei to the newly formed 

 neuroblast-nuclei be further studied several significant facts with 

 reference to the later phases of nuclear division manifest themselves. 

 For example, it will be found that when the metaphase of mitosis, as 

 exhibited in Fig. 1, gives place to the kataphase, the relative positions 

 assumed by the daughter chromosomes in the resting nucleus can 

 be readily elucidated. Thus the elongated nucleus in Fig. 1 has 

 probably been recently produced by mitosis, and it will be noted 

 how remarkably bipolar it is. Therefore the centrosome in relation 

 to it must have at first assumed a position somewhere near the centre 

 of one lateral margin. The neighbouring kataphase in Fig. 1 gives 

 the clue to this, for the daughter-nuclei to be derived from that mitotic 

 figure will take up a position alongside the elongated resting nucleus. 

 Further, it will be observed that the long axes of the daughter-nuclei 

 lie in a plane at right angles to the direction of separation of the 

 karyokinetic figure. We thus become confronted with a subtle 

 problem in bio-dynamics in relation to the phenomena of karyo- 

 kinesis. 



During the early developmental stages the neuroblast-nuclei in 

 the outer layers of the retinal wall retain their radial arrangement, 



iQwing to the invagination of the optic vesicle its lining membrane becomes the 

 external limiting membrane of the retina. 



