78 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



Visual Organs. — The sensory portion of the eyes arises from 

 the brain, and in the embryos of some vertebrates (elasmo- 

 branchs, urodeles) optic areas can be recognized in the medullary 



Fig. 84. Diagrams showing the inversion of the layers in the formation of the 

 retina. In all the figures the nuclei are placed in the morphologically deeper ends 

 of the cells. In A the brain (^) has been closed in, in B the optic vesicle {y) has 

 reached the thickening for the lens (/), and on the right side the vesicle is being con- 

 verted into the double-walled cup with, as shown in C, a medial epithelial (?), and 

 an outer retinal layer (^), the deeper face of which is turned towards the lens. 



plate before its involution. The accessor}' portions are furnished 

 by ectoderm, mesothelium, and mesenchyme. 



A hollow outgrowth arises on either side of the primitive 

 fore brain, and extends towards the skin. The distal portion 



expands into a globular optic 

 vesicle, while the proximal por- 

 tion retains its smaller size, and 

 is known as the optic stalk. 

 Thus the cavity of the vesicle 

 is in connection with the ven- 

 tricle of the thalamencephalon 

 by means of the hollow stalk. 

 The distal surface of each 

 optic vesicle conies in contact 

 with the ectoderm of the side 

 of the head at the place where 

 the lens is to form (see below), 

 and with the formation of this 

 structure the distal half of the 

 optic vesicle becomes invaginated into the proximal part, thus 

 partially obliterating the cavity of the vesicle, and converting the 

 whole into a two-layered cup. The distal invaginated part of 



Fig. 85. Diagram of early develop- 

 ment of eye, modified from Hertwig. h, 

 blood-vessels ; < , cavity of nptic vesicle ; 

 e, epithelial layer ;/, choroid fissure; /, 

 lens; r, retinal layer: .■, cavity of optic 

 cup, later occupied by vitreous body. 



