266 JAMES ROLLIN SLONAKER 



downward from the brain cavity. The right vesicle shows a 

 noticeable thickening of its external wall and the adjacent 

 ectoderm (L). 



The ectoderm now begins to dip in from the outside (text- 

 fig. 2, B, I.) and at the same time pushes the outer wall of 

 the optic vesicle before it. There is thus formed the double- 

 walled optic cup. This stage is reached in the chick at about 

 the age of fifty hours. Figure 44 represents a section through 

 the head of a chick of fifty-six hours' incubation. The invagi- 

 nation of the ectoderm, Lc, and the formation of the optic cup 

 is very noticeable. At this age the optic vesicle is reduced to a 

 shallow cavity and there is marked difference in the thickness of 

 the two walls, the anterior wall being much the thicker. 



The edges of the ectodermal invagination approach each 

 other and finally unite to form the lens vesicle. Later this 

 separates completely from the ectoderm which forms a continuous 

 layer over it (text-fig. 2, C, 1). As development proceeds the 

 optic cup becomes deeper and deeper until the two walls (text- 

 fig. 2, r and Pg) are in contact and the cavity of the primary 

 optic vesicle is wholly obhterated. The inner layer (r) rapidly 

 increases in thickness and finally develops into all the different 

 layers of the retina, excepting the pigment layer which develops 

 from the outer wall [Pg) . 



A microphotograph of a section through the head of a sixty- 

 four-hour chick is given in figure 45. At this age the lens vesicle 

 [Lc) is completely formed and separated from the ectoderm, 

 which appears as a dark line immediately in front of and adjacent 

 to the lens vesicle. Owing to the hardening process, the optic 

 cup is somewhat distorted, but is seen to be well formed. The 

 retinal portion of the cup {R) is several times thicker than the 

 posterior pigment portion (P) . 



Sections in a vertical plane show that all portions of the wall 

 of the optic vesicle do not grow at the same rate. The cells on 

 the dorsal side multiply more rapidly than the others and soon 

 produce an unsymmetrical shape. A diagram of a vertical 

 section through these parts is shown in text-figure 3, A. This 

 view would be obtained, were a section made through the devel- 



