■U BULLETIN OF THE 



The first traces of the eyes in the development of the lobster are the 

 optic disks. These disks lie one on either side of the median plane, and 

 are for a considerable time the most conspicuous structures in the an- 

 terior region of the embryo. In the early stages of development the 

 disks face ventrally, but as the head of the animal becomes differen- 

 tiated they come to face almost in the opposite direction, i. e. dorsally. 

 At stage A (Fig. 37) the optic disk, so called, is oval in outline rather 

 than disk-shaped ; its longer axis is transverse to the principal axis of 

 the embryo. From that portion of each disk which is near the median 

 plane a band of tissue extends posteriorly, and connects the disk with 

 the ventral plate of the embryo. The disk and the band with which it 

 is connected to the ventral plate are distinguished from the surrounding 

 tissue by their greater number of nuclei. The disks comprise the tissue 

 from which both retina and optic ganglia develop. 



A section passing through an optic disk in a plane perpendicular to 

 the longitudinal axis of the embryo is shown in Figure 38. In this 

 case the section is from the right optic disk. The left side of the sec- 

 tion is farthest from the median plane ; the right side is near that 

 plane. Since at this stage the disk faces ventrally, and since the ventral 

 edge of the section is uppermost in the figure, it is the posterior face of 

 the section which is presented. To the right and left of the disk one 

 can see the undifferentiated ectoderm with its occasional nuclei. This 

 ectoderm is directly continuous with the tissue of the disk ; in fact, the 

 disk is only a local thickening in the ventral ectodermic layer of the 

 embryo. It is due to its greater thickness that the disk contains more 

 nuclei than the surrounding ectoderm. The deep face of the ectoderm, 

 both of that which is undifferentiated and that which forms the disk, is 

 limited by a delicate but distinct basement membrane (Fig. 38, mb.). 



Some idea of the method of growth in the optic disk can be gained 

 from a study of its nuclei. It will be noticed that the nuclei in the 

 deeper part of the disk are rather small and irregularly grouped when 

 compared with those which are found next its outer surface. These 

 superficial nuclei form an almost regular series, which extends from one 

 margin of the disk to the other. They are usually also characterized by 

 having their long axes perpendicular to the surface of the disk. When 

 they increase by division, their planes of separation are in most cases 

 either parallel or perpendicular to the outer surface of the disk. When 

 the plane of separation is perpendicular to the surface of the disk, the 

 tendency for such divisions is to increase the diameter of the disk. 

 When, on the other hand, the plane of separation is parallel to the 



