330 ORR. " [Vol. I. 



period, and it seems to take no part in the dorsal enclosure of 

 the fore-brain. 



If we imagine a middle longitudinal axis running through the 

 neural tube of this earliest stage, its anterior part would have a 

 marked ventral and slightly posterior curve,^ and would end at 

 a median point on the inner surface of the anterior fold. The 

 growth of the lateral walls continues chiefly in a dorsal direc- 

 tion, so that before their dorsal fusion takes place the lateral 

 folds are about twice as high from the floor as is the anterior 

 fold. This I have ascertained by a series of horizontal sections 

 parallel to the axis of the fore-brain in a slightly older embryo 

 of SphcBrodactylns notalus. The dorsal growth of the fore-brain 

 brings its middle longitudinal axis into such a position that its 

 anterior end at the time of the dorsal fusion is a point about 

 the dorsal edge of the anterior, fold. For convenience sake I shall 

 consider the axis as ending exactly at this spot. Fig. 45, PI. 

 XV., represents a section through the fore-brain of an embryo 

 of sphserodactylus slightly older than the embryo of series A. 

 This section runs just dorsal to the longitudinal middle axis of 

 the fore-brain, and parallel to the same. It is also symmetrically 

 horizontal to the back of the embryo ; but this part is assym- 

 metrical because of the lateral twist of the head. The fore-brain 

 is not yet dorsally enclosed ; a narrow slit (c*) extends from the 

 dorsal edge of the anterior fold to the division between fore- 

 and mid-brain. The sections of this series which are ventral to 

 the longitudinal middle axis show that part of the lumen of the 

 fore-brain which is inclosed anteriorly by the anterior fold. 

 The latter, at this stage, is of the same appearance and thick- 

 ness as the lateral folds. Later, the floor of this part of the 

 brain becomes the infundibulum. The rudiments of the optic 

 vesicles {Opv) are comparatively very large. In the figure, the 

 vesicle of only one side is seen ; on the other side the section 

 passes dorsal to the vesicle. The distal end of the vesicle bends 

 slightly in a posterior direction. In the dorsal half of the fore- 

 brain — i.e., that part above the middle axis and the edge of the 

 anterior fold — the anterior walls of the two optic vesicles are 

 separated from each other by the narrow slit o. This slit, there- 

 fore, extends the entire dorsal length of the primary fore-brain, 

 and down the anterior surface to a point between the rudiments 

 ' The lateral twist of the head is here not taken into accoant. 



