DEVELOPMENT OF THE CEEEBRO-SPINAL AXIS. 293 



The origin of the spinal cord and brain is illustrated in the annexed 

 figures from Bischoff. Fig. 136 shows upon a dark ground Ori . Q of thg 

 a portion of the germinal membrane, in the midst of which is spinal cord and 

 the area pellucida and primitive trace : #, the area pellucida ; the bram> 

 , the dorsal laminse ; c, the primitive trace. 



Fig. 136. Fig. 137. 



The primitive trace, magnified 8 diameters. Origin of the brain upon the spinal cord, magnified 



8 diameters. 



Fig. 137, the same at a later stage, preparation for the brain being 

 made. The dorsal Iamina3 are approaching each other, particularly to- 

 ward the middle: , the dilated upper extremity or cephalic end, the 

 three cells appearing : the epencephalon, mesencephalon, and deutenceph- 

 alon ; b, chorda dorsalis along the bottom of the groove ; c, rudiments of 

 vertebra ; . d, lancet-shaped dilatation. In both figures the pale borders 

 along the primitive trace are pellucid nerve substance. 



The dorsal cord, which is only a transitory structure, now disappears, 

 the spinal marrow commencing to exhibit a division into four strands, 

 right and left, upper and under. The medulla oblongata flattens next 

 in its upper part, its fasciculi parting from each other; the interval so 

 arising between them is to be the fourth ventricle. The hemispheres 

 now appear as a double cell, the prosencephalon, and as development goes 

 on, they soon exceed the corpora quadrigemina in size, and, as they ad- 

 vance, force these bodies backward and under them. 



From this it appears that the type of construction of the nervous sys- 

 tem is, that upon the rudimentary spinal marrow a series of vesicles is 

 developed. They constitute eventually the medulla oblongata, the cer- 

 ebellum, the corpora quadrigemina, the thalami optici, the corpora striata, 

 the olfactive ganglia, and in front of all, but destined to cover the anterior 

 portions over, the hemispheres. 



Turning now to the animal series, we find in the lowest members of 



