342 ORR. " [Vol. I. 



said, the ventral transverse commissure does not appear in the 

 fore-brain. The main part of the lateral band, following the 

 curve of the cranial flexure, passes along the ventral lateral wall 

 of the fore-brain, and appears on the anterior surface of the fore- 

 brain immediately ventral to the optic stalks. Fig. 52, E, PI. 

 XV., shows a section of the fore-brain cut at right angles to its 

 anterior surface and immediately ventral to the optic stalks. 

 The lateral band i^LF) bends slightly dorsal to the plane of this 

 section, and then, entering the plane again, passes in it around 

 the anterior surface of the brain (^LF"). At LF" the lateral 

 longitudinal fibres unite, blending together so that this whole 

 system may be described as a band of generally parallel fibres 

 extending across the anterior surface of the central nervous 

 organ and running posteriorly along its sides. At the stage rep- 

 resented in Fig. 53, E, there is only one other band of fibres in 

 the region of the primary fore-brain, and that is the posterior 

 commissure {PCs). Its general appearance is as if, on en- 

 tering the fore-brain, a part of the fibrous mass had split off 

 dorsally, and instead of following the curve of the cranial flex- 

 ure these fibres continued across the dorsal surface of the brain 

 just anterior to the mid-brain. In Fig. 52, E, the posterior 

 commissure is cut obliquely near its point of separation from 

 the lateral band. In Fig. 53, E (a section dorsal and anterior to 

 52, E), is shown the posterior commissure crossing the dorsal 

 surface of the brain. Fig. 54, E, shows the first rudiment of 

 the pineal eye, which arises a little distance anterior to the pos- 

 terior commissure. Fig. 55 shows a section from an embryo of 

 about the same stage as that of series E. This passes through 

 the hypophysis {Hph) and fore-brain parallel to the anterior 

 surface of the latter, cutting also the optic stalk (O.st), (cf fig- 

 ures of series D). Immediately ventral to the optic stalk is 

 seen the lateral band of fibres — in cross-section — just poste- 

 rior to their meeting on the anterior surface of the brain. Out- 

 side the region of the bands above described no trace of fibres 

 can be found in the brain at this stage ; nor have any fibres ap- 

 peared yet in the optic stalks. By comparing this description 

 of the main band of longitudinal fibres with the description of 

 earlier stages, it becomes evident that the main band of fibres 

 follows the course of the primitive lateral and anterior walls of 

 the medullary groove or tube. The band occupies a position 



