254 



HISTOLOGY 



layer, instead of being invaginated from the site of the future eye, is 

 invaginated from a wall of the neural tube which lies under the site of 

 the future eyes ; thus it is seen that the retina is a part of the brain's wall 

 (Fig. 224, A}. It must be remembered in this connection that the brain 

 tube was itself an invagination of the original ectoderm. 



The neural invagination reaches toward the skin in a cup-like form, 

 and at the same time a thickened area of this skin forms a depression 



FIG. 224. Five sketches to represent five stages in the development of the rabbit's eye. (From 

 "STOHR'S Text-book of Histology" by LEWIS.) 



which advances inward to meet it (Fig. 224, B}. This latter structure 

 becomes the lens by being constricted off as a sac from the external 

 epithelium (Fig. 224, C), and undergoing a thickening of its posterior 

 wall of epithelium, until this wall fills the sac up solid with its long, paral- 

 lel, fiber-like cells. Its nuclei thus form a row across the middle of a 

 section of the lens (Fig. 224, D, ), and the anterior wall becomes a 

 layer of simple epithelium covering the anterior surface. 



The edges of the optic cup, as the brain invagination is called, embrace 



