52 PROF. W. K. PAEKER ON THE 



in a semicircular notch on the back of the interorbital wall, a little in front of and 

 above the deep pituitary space {py.). The lesser nerves, 3rd, 4th, and 6th, pass out to 

 their destinations through the membranous interspace that is now seen between the 

 foramen opticum and the f. ovale ; the latter is very large for the transmission of the 

 5th nerve (PI. VIII. fig. 2, v.). It lies over the meatus internus for the 7th and 8th 

 nerves (vii., viii.). The passages for the vagus, glossopharyngeal, and hypoglossal 

 nerves (PL VIII. fig. 1, x., xii.) are figured in the least and largest of these embryos. 

 The glossopharyngeal and vagus pass out under the opisthotic bone, and the hypo- 

 glossal pierces the exoccipital. In the Sauropsida there is a small passage in front of 

 it which makes it seem to be the " posterior condyloid foramen ; " in Mammals a 

 notable vascular passage is found behind that for the 12th nerve. That nerve in the 

 bird (see PI. VIII. fig. 2, xii.) is shown as escaping opposite the occipital condyle, with 

 its notochordal dimple {nc), and having directly in front of it a small vascular passage. 

 The large internal carotids find their way into the skull over the parasphenoid and pass 

 through the pituitary hole ; they ultimately have a bony tube formed accurately round 

 them. The rest of the cranium proper is formed of membrane and membrane-bones, 

 or parostoses. 



But besides the inner skull, or chondrocranium, there are the cartilaginous visceral 

 arches that are formed in segments in that tract of the head of the embryo which 

 corresponds with the splanchnopleura in the postcephalic region. These cartilages 

 begin to solidify before the head-cavities are closed ; they are indeed the first three 

 branchial arches ; for, as is well known, in the Elasmobranchs the quadrato-mandi- 

 bular and hyoid arches both carry gills L 



Whatever the prochordal tracts of the chondrocranium may turn out to be, whether 

 visceral or cranial, the post-oral arches are determinable ^. 



In this type, as in most of the Sauropsida, the palato-pterygoid arcade in front of 

 the quadrate pier is merely developed as membrane-bone, although secondary tracts of 



' If this fact had been attended to, the strange misconceptions which have arisen as to the nature of these 

 parts would have been avoided. In the study of the Morphology of the Skull, before all things, it is necessary 

 that the fundamental embryological development of this part should be mastered. Anything more hopeless 

 than the confusion produced by want of this knowledge in some Memoirs on this subject cannot be conceived. 

 The most remarkable instance of this is to be seen in Dr. Gadow's paper " On the Modifications of the First 

 and Second Visceral Arches " (Phil. Trans. 1888, pla. 71-74, pp. 451-485). If our modern Morphology can 

 do no more for us than this, we had better return to " Transcendentalism " and blindly follow Oken. 



- Prof. Huxley (see his paper on Petromyzon, Journ. Anat. & Phys. vol. x. pp. 412-429) classifieB the 

 trabeculse with the visceral arches ; my own view at present is that they do belong to the same original 

 cranio-facial basket-work, such as is seen in the Myxinoids (Phil. Trans. 1883, pis. viii.-xxvi.) ; hut that the 

 whole of that continuous skeletal tract is, I feel certain, a prochordal skeleton formed in time before any 

 vertebral rudiments appeared, and even before there were any skeletal elements in the occiput. This basket- 

 work was developed for the support of the enlarging brain and increasingly complex oral apparatus long before 

 any paired vertebral rudiments grew for the support of the notochord. 



