No. 2.] EMBRYOLOGY OF THE LIZARD. 343 



corresponding to the external surface of these walls. In the 

 spinal cord this identity of parts is at once evident; but in 

 the head the dorsal dilatation of the primitive tube to form the 

 swellings of the brain has left the band in a position relatively- 

 more ventral. 



In the stages immediately succeeding that which has just been 

 described, the fibres of the optic nerve make their appearance. 

 Hoffman has stated that the fibres of the optic nerve form first 

 on the ventral wall of the optic stalk. This is true according 

 to a general and rather inexact terminology ; but it is on the 

 morphologically anterior wall of the optic stalk that the fibres 

 fi.rst appear. The fibres of the opticus develop in a manner 

 very different from the fibres of all the other nerve-roots. They 

 are not polar elongations, nor do they originate in the internal 

 ganglion-cells of the neighboring brain-wall. They are formed 

 next the external surface of the anterior wall of the optic stalk 

 and the lateral wall of the optic cup, in the same manner as the 

 fibres of the lateral bands are formed. The fibres appearing on 

 the anterior wall of the optic stalk are continuous with the dor- 

 sal edge of the anterior and lateral band above described, so 

 that the fibres of the band appear to continue outward along 

 the stalk. Fig. 61 shows a horizontal section through the optic 

 stalk at right angles to the anterior surface of the primary fore- 

 brain. As the stalk does not lie exactly in a horizontal plane, the 

 section cuts it obliquely. The stalk still has a lumen (/), and on 

 its anterior wall i^op"^ are the nerve-fibres. There is no trace 

 of fibres on the posterior wall {op'). At this stage I could not 

 trace any decussation of the fibres from the opposite sides, — 

 probably because the fibres run so nearly parallel. Fig. 60 

 represents a section through the anterior part of the eye-cup 

 and part of the optic stalk, in the same plane as Fig. 61. This 

 section shows the fibres NF developing on the anterior wall of 

 the stalk {op") and on the lateral wall of the eye-cup {Ey'). 

 The fibres in the eye-cup do not spread equally in all direc- 

 tions, but extend chiefly in a posterior direction, which is the 

 direction of the long axis of the cup. The lateral wall of the 

 optic cup has increased very much in size ; while the median 

 wall has become very thin, and is in some places a mere mem- 

 brane. The corresponding walls of the optic stalk are affected 

 in a similar manner. The anterior wall, as it develops the 



