MUSEUM OF COMPARATIVE ZOOLOGY. 35 



surface of the disk, the tendency is to thicken it. In order to determine 

 the distribution of these two methods of division, the planes of sepa- 

 ration in the superficial nuclei of four disks were carefully observed. 

 Each disk can be divided into halves by a plane parallel to the sagittal 

 plane of the embryo. That half of the disk which lies near the median 

 plane can be called the proximal half; that which is farther from the 

 median plane, the distal half. Among the superficial nuclei of the distal 

 halves of the four disks which were examined there were seen thirteen 

 cases of division. In all of these cases the tendency was to broaden the 

 disk. There was no case where the division of the nucleus would have 

 thickened it. In the proximal halves of the disk there were six cases 

 of division observed ; five of these tended to thicken the disk, and one 

 would have broadened it. It is therefore apparent that in the super- 

 ficial layer of each disk the proximal half is becoming thicker, while the 

 distal half is becoming broader. 



The deep nuclei of each disk, those lying below the row of superficial 

 nuclei, divide in different planes. Among these nuclei in the four disks 

 which were examined, seven instances of division were noticed. Five of 

 these were in planes which would have thickened the disk ; the remain- 

 ing two would have broadened it. This part of the disk consequently 

 shows a tendency both to become broader and thicker. Of these two 

 tendencies, that which would thicken the disk is the stronger. 



The method of growth which has been described for different parts of 

 the optic disk can be easily distinguished only in its earlier stages. The 

 subsequent changes which affect the structure of the disk render it 

 rather difficult to follow in detail the growth of the disk as a whole ; but 

 in general the broadening of the distal superficial region, the thickening 

 of the proximal superficial part, and the broadening and thickening of 

 the deeper parts are continued. 



The most important changes in the differentiation of the optic disks 

 are the following : first, the separation of the retina and optic ganglion 

 by the formation of the basement membrane ; second, the production of 

 the optic nerve ; and third, the differentiation of the ommatidia. These 

 three changes will be considered in the order named. 



The first step in the differentiation of the retina and optic ganglion 

 occurs at stage B. Figure 39 represents a section from the optic disk of 

 an embryo of this stage. The plane of section in this case corresponds 

 to that of Figure 38. The chief difference observable between the disk 

 at stages A and B is its greater thickness in the older embryo. Not only 

 has the disk thickened, but it has spread laterally, so that the small 



