No. 3.] A HUMAN EMBRYO TWENTY-SIX DAYS OLD. 463 



Central Nervous System. 



When the neural tube is straightened it measures from end to 

 end 17 mm. with a diameter of 1.5 mm. for the fore-brain, and 

 0.5 mm. at the point between the posterior extremities. Its 

 general shape is shown, Plate XXX., which corresponds quite 

 well with His's Br. z } On the exterior it is plainly shown that 

 the cerebrum and optic vesicle are attached to the fore-brain. 



The cerebral hemispheres are represented on both sides by 

 oval projections from the fore-brain, extending somewhat over 

 the surrounding tissue on the mid-brain side, and under the eye. 

 Viewed from the median side the pit represents a stage midway 

 between His's Br. 3 and K. O. 2 On the median line between 

 the two hemispheres there is a fold of medullary tube-wall which 

 extends more than a third across the cerebrum, and has a ten- 

 dency to cut it in half. It measures 0.4 mm. in perpendicular, 

 and projects 0.16 mm. into the fore -brain vesicle. Compared 

 with His's figures, this is undoubtedly the epithelial covering 

 of the choroid plexus. 



Towards the mouth from the cerebrum there is the opening 

 into the optic vesicle. It is triangular in shape, with the base 

 on the oral side, and the apex pointing towards the inter-brain. 

 The opening within the stem of the optic vesicle is round, and 

 ends as a circle about the secondary ocular vesicle. Viewed 

 from the outside, the secondary ocular vesicle is a round pit, in 

 which lies the lense. There is no slit on the oral side of the 

 stem (Plate XXX., Fig. 1). 



The inter-brain shows a marked constriction in its middle, 

 both on the outside and also within its lumen. This undoubt- 

 edly was caused by a shrinkage when the specimen was hardened. 



As the mid-brain and hind-brain are approached, the walls 

 gradually become thicker and thicker, until the after-brain is 

 reached, when the ventral side alone increases, while the dorsal 

 walls become very thin. From the origin of the facial nerve to 

 the origin of the pneumogastric, the walls of the dorsal half 

 of the neural tube are very thin. From now on, the thickness 

 of the walls of the tube, with the exception of the extreme dorsal 



1 Abhandl. d. K. S. Ges. d. Wiss., Bd. XIV., 1S88. 



2 Abhandl. d. K. S. Ges. d. Wiss., Bd. XV., Figs. 2 and 5. 



