THE CEREBRAL HEMISPHERES. 401 



distance prolonged. This solidification is arrived at, so far as I 

 have determined, without the intervention of a fold. The 

 nerves are fibrous, and a commencement of the chiasma is 

 certainly present. From the chiasma there appears to pass out 

 on each side a band of fibres, which runs near the outer surface 

 of the brain to the base of the optic lobes (mid-brain), and here 

 the fibres of the two sides again cross. 



By stage O important changes are perceptible in the cerebral 

 rudiment. In the first place there has appeared a slight fold at 

 its anterior extremity (PI. 16, fig. 3, x), destined to form a 

 vertical septum dividing it into two hemispheres, and secondly, 

 lateral outgrowths (vide PI. 16, fig. 2, ol. I), to form the olfactory 

 lobes. Its thin posterior wall presents on each side a fold which 

 projects into the central cavity. From the peripheral end of 

 each olfactory lobe a nerve similar in its histological con- 

 stitution to any other cranial nerve makes its appearance (PI. 16, 

 fig. 2) ; this divides into a number of branches, one of which 

 passes into the connective tissue between the two layers of 

 epithelium in each Schneiderian fold. On the root of this 

 nerve there is a large development of ganglionic cells. I have 

 not definitely observed its origin, but have no reason to doubt 

 that it is a direct outgrowth from the olfactory lobe, exactly 

 similar in its mode of development to any other nerve of the body. 



The cerebral rudiment undergoes great changes during stage 

 P. In addition to a great increase in the thickness of its walls, 

 the fold which appeared in the last stage has grown backwards, 

 and now divides it in front into two lobes, the rudiments of the 

 cerebral hemispheres. The greater and posterior section is still 

 however quite undivided, and the cavities of the lobes (lateral 

 ventricles) though separated in front are still quite continuous 

 behind. At the same time, the olfactory lobes, each containing 

 a prolongation of the ventricle, have become much more pro- 

 nounced (vide PI. 1 6, figs. 40 and 4^ ol.l). The root of the 

 olfactory nerve is now very thick, and the ganglion cells it con- 

 tains are directly prolonged into the ganglionic portion of the 

 olfactory bulb ; in consequence of which it becomes rather 

 difficult to fix on the exact line of demarcation between the bulb 

 and the nerve. 



Stage Q is the latest period in which I have investigated the 



