1879.] 



Skull and its Nerves in the Green Turtle. 



343 



On the Organic and Actual Fore End of the Head. 



When the cephalic fold of the embryo is formed, the fast-growing 

 cerebral bulbs hang over the yolk; the brain at this stage, by its own 

 bulk and weight, hangs down like a gourd. 



Of necessity, this throws together parts that would be at some con- 

 siderable distance from each other; also the organic end is not the 

 actual end of the head, as I have mentioned before : that is formed by 

 the mid-brain ; the fore-brain, or terminal vesicle, looks downwards 

 and backwards. 



In this piece of morphology, as in studying the " receptacle of the 

 fig-tree," we have to distinguish between the apparent apex and the 

 true apex. 



But if organs have to be supported, the morphological force must 

 make good that which is thrown out of gear by this special hyper- 

 trophy of parts", and the beams and rafters must be eked out. 



For this heavy nodding bend of the growing brain so doubles up 

 the undergrowths of the skeletal rudiments, that both the paired and 

 unpaired parts are stopped very near to the retral organic apex. 



Also the nasal sacs and optic vesicles, attached to, or growing from, 

 the fore-brain, are in a new position, and not, as they should be, in 

 the old straight line. 



Indeed, the axis of these skeletons grows up far into the hollow 

 bend of the mid-brain, and the notochord turns over somewhat, and 

 then loses itself in the mesoblastic sheath, which gets very close to 

 the organic end of the brain. 



The paired tracts (para- chor dais, both trabecular and post-tra- 

 becular) grow up into that hollow, and there stop ; their new out- 

 growths (or eking out of the basi-neural tracts) begin again at the 

 base of this ascending wall. 



Also the middle (unpaired') part begins again directly, in Ghelone, 

 and in embryos (two- thirds ripe), in which the notochordal sheath 

 was ossifying to form the beginning of the " basi-occipital " and 

 " basi-sphenoid," the cephalostyle passed at once from the noto- 

 chordal sheath to embrace and ossify the " inter- trabecular " bar, 

 close below the pituitary body. 



That part of the basi-sphenoid which lies in front of and below the 

 pituitary body is formed by ossification, directly from the cepha- 

 lostyle, of the unpaired solid pro-chordal tract of mesoblast ; after- 

 wards the bony matter spreads into the paired bars (trabecule). 



But that middle bar came short of the pituitary space in the earlier 

 stages ; for there the oral fold grew up to graft itself upon the fore- 

 brain, close behind (or below, organically), the true apex or end of the 

 brain. 



Hence we see, that if the pituitary body were formed in an unbent 



