138 



EYE 



opening out of the optic cup a point the significance of which will appear later. 

 In the few cases in which an open lens-pit has been described in the human 

 embryo it is figured as a thick-walled, cup-shaped, then flask-shaped depression, 

 the lips of which come together to form a vesicle which is connected for a time 

 with the surface ectoderm by an epithelial stalk, but afterwards becomes com- 

 pletely separated from it. The inner wall of the vesicle at an early stage 



FIG. 181. DEVELOPMENT or THE LENS IN THE BABBIT. (After Rab], from Hertwig's 

 Handbuch der Entwickelungslehre.) 



Nos. 1 to 8 x 130 diameters ; No. 9 x 91 diameters. The stages 1 to 5 embryos from the middle of the 

 eleventh to the middle of the twelfth day. No. 6 an embryo at the end of the twelfth day. 



increases in thickness and encroaches on the cavity, while the outer remains a 

 thin lamella (fig. 181). 



In the rabbit embryo the lens-pit does not coincide with the centre of the lens-plaque ; 

 therefore in cross-section it is triangular, not hemispherical. The vesicle consequently is also 

 triangular in section, and it is rather the upper and inner wall which becomes thickened by 

 the elongation of its cells. The lens in its early stages thus appears in cross-sections of the 

 embryo to be obliquely placed in the optic cup (fig. 181). It may also be mentioned, though 

 the significance of the fact is unknown, that in the rabbit, and also in man (Rabl), the vesicle 

 contains a mass of epithelial cells, which undergo degenerative changes and ultimately disappear. 





