178 THE CEREBRAL HEMISPHERES. 



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 important changes are perceptible in' the cere- 

 bral rudiment. In the first place there has appeared a slight 

 fold at its anterior extremity (PL XV. fig. 3, x), destined to form 

 a vertical septum dividing it into two hemispheres, and secondly, 

 lateral outgrowths (vide PI. xv. fig. 2, oil), to form^ the ol- 

 factory lobes. Its thin posterior wall presents on each side a 

 fold which projects into the central cavity. From the peri- 

 pheral end of each olfactory lobe a nerve similar in its histo- 

 logical constitution to any other cranial nerve makes its appear- 

 ance (PL XV. 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 tlie last stage has grown back- 

 wards, 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 i\iQ same time, the olfactory lobes, each 

 containing a prolongation of the ventricle, have become much 

 more pronounced (vide PL xv. fig. 4a and 4c, oil). The root of 

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

 contains 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 development of the brain. Its structure is represented 



