344 ORR. ■ [Vol. I. 



fibres, increases In thickness. The posterior wall develops no 

 fibres, but gradually becomes thinner. At a period when the 

 two walls are still distinct, though the lumen has been obliter- 

 ated by the growth of the anterior wall, the posterior wall is a 

 thin layer of flattened cells, connecting the membraneous wall 

 of the eye-cup and the cellular, non-fibrous brain-wall imme- 

 diately dorsal to the lateral band. This thin layer of flattened 

 cells is entirely free from fibres, and in stages a little older it has 

 disappeared, excepting a wedge-shaped cellular projection of 

 the brain-wall immediately posterior to the point where inter- 

 nally a conical depression marks the original median opening 

 of the stalk-lumen (/), (cf, Fig. 6i). The dorsal and ventral 

 division of the anterior and posterior walls of the optic stalk 

 corresponds originally with the line along which the lateral and 

 median walls of the optic cup unite. 



During the period of the development of the optic-nerve 

 fibres and the formation of the chiasma, the brain undergoes 

 considerable change. The parts of the brain already present 

 become more pronounced, and the lobes of the secondary 

 fore-brain appear. These changes are illustrated in Fig. 61, F, 

 Anteriorly this section passes to the right of the fold which 

 separates the ventricles of the two lobes (see description of 

 figure). The dorsal anterior part of the primary fore-brain 

 becomes dilated into a small, rounded swelling (/^5'', Fig. 6). 

 The anterior surface of this round dilatation extends to a point 

 just dorsal to the optic stalks, where its boundary is marked by 

 a constriction. A little later a small median groove appears in 

 the roof of the swelling, dividing it into lateral halves. From 

 this time on, each half continues its growth separately, leaving 

 the originally slight groove as a deep cleft separating the two. 

 Bearing in mind the foregoing descriptions, it is evident, from a 

 comparison of Fig. 51, D, with Fig. 63, F, that the secondary 

 fore-brain is a dorsal and not an anterior outgrowth. In this 

 latter stage (63, F) the increase of the cranial flexure has 

 brought the infundibular region relatively much nearer the floor 

 of the hind-brain. The secondary fore-brain is shown at its 

 largest vertical and longitudinal diameters in this figure. 



Fig. 62, F, represents a section of the same series as 63, F, 

 and in a parallel plane, but passing through the lateral wall of 

 the brain. This section shows the lateral longitudinal band of 



