1879.] Skull and its Nerves in the Green Turtle. 



333 



Between these there is another (azygous) rod ; this passes between 

 the trabecule behind, it lies on a lower plane, and ends where the front 

 of the pituitary body will be, for this part is only forming as yet. 



In front this rounded rod runs forwards to the anterior wall of the 

 head ; not so the trabecule, for they, unlike what is seen in the 

 Batrachia, are behind the nasal sacs, and never pass far into that 

 region. 



In the frog, and his congeners, they form a broad floor to the nasal 

 labyrinth. 



At any earlier stage than this the notochord with at least a film of 

 mesoblast, ensheathing the intrinsic sheath, might have stretched itself 

 to the point where this pre-pituitary rod ends behind ; but it took an 

 upward course, and, following the curve of the mid-brain, turned 

 down again, and then stopped short. 



I cannot see what other explanation of this solid rod can be given 

 than that it is the true homologue of the mesoblastic cartilaginous 

 sheath of the notochord ; it is solid because it has nothing to enclose. 



Mr. Balfour is quite right in saying that the mesocephalic flexure is 

 only apparently recovered from ; the head, indeed, has its axis per- 

 manently shortened by this bend of the mid-brain on itself. 



Indeed, the skull is shortened by this, as a dress is shortened by 

 having a " tuck " taken up in it ; or as a river shortens itself when it 

 cubs out a new channel at the base of a sharp bend. 



The " post-pituitary wall " lies in the axis of the arched space formed 

 by the bend of the mid-brain on itself ; it is extremely large in the 

 embryo of the green turtle. 



It must be considered that the fronta I wall is not the organic end of 

 the embryo, but the upper surface of that end. 



The fore-brain looks directly downwards, and even a little back- 

 wards ; the olfactory nerves arise from its anterior ( = superior) sur- 

 face, and the " infundibulum " buds out to meet the oral involution 

 on the posterior (= inferior) surface of the fore-brain. 



We thus see that the true organic punctum terminate must lie 

 between these two parts, the olfactory nerve and infundibulum, and 

 therefore it looks downwards, and a little backwards. 



So when the trabecule cranii grow to the frontal w all they in reality 

 turn upwards, and embrace the dorsal region of the front of the head. 



Thus, it is evident that if we can trace the notochord to the back 

 of the pituitary body, we have found it reaching, very nearly, the 

 fore- end of the embryo. 



Now, in my third stage of Chelone midas the notochord turns round 

 in the post-clinoid upgrowth of the basal plate, and the sheath in its 

 descending part becomes solid, and ends behind the lobules of the 

 rudimentary pituitary body as a tear-shaped drop or lump of car- 

 tilage. 



