440 J. (UlAHAM KERR. 



downwards to the ventral side of the brain (fig. 12, C). The 

 brain being also meanwhile greatly increasing in depth, the 

 optic rudiment is caused to slope upAvards to reach its point 

 of contact with the skin. 



Later still, with the active growth in length of the brain 

 rudiment, the point of attachment of the optic stalk becomes 

 carried far forwards from the level at which its end portion 

 is, so to speak, tacked on to the skin. The optic nerve now 

 passes from its origin backwards to a great extent before 

 reaching the skin. 



Already in Stage 25 the wall of the optic vesicle lying 

 next the skin begins to show a thickening to form the retina 

 (fig. 12, D), and at the same time, or very slightly later (fig. 

 12, F), a slight thickening in the external epidermis fore- 

 shadows the formation of the lens. The retina rapidly 

 thickens (fig. 12, G), assumes a convex surface internall}^, 

 and by Stage 30 fits closely to the surface of the inner wall 

 of the vesicle (fig. 12, H). 



The lens, at first a simple thickening of the deep layer of 

 the epiblast, begins to develop a cavity in its interior about 

 Stage 30 (fig. 12, H). It develops somewhat ventrally to 

 the optic vesicle, and there is at first a wide space between it 

 and the ventral rim of the cup. 



As usual the invagination takes place partly from below, 

 and in Stage 30 there is a wide choroid fissure, involving, 

 however, only the retinal cup. The fissure closes remark- 

 ably soon in Lepidosiren. Already by Stage 31 it has 

 been obliterated. 



In embryos a little older than Stage 311 find traces only 

 persisting; in one case a depression in the rim of the cup, 

 in another mesoblast perforating its wall. The choroid fissure 

 being of such a transitory nature is by no means so con- 

 spicuous a feature as we arc accustomed to in the higher 

 vertebrates. 



The development of the lens from Stage 31 onwards seems 

 to pursue a normal course, which is sufficiently indicated by 

 fig. 12, I, J, etc. 



