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, E), 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 



