Section 33. Besides these great differences in form, there are important differences in the amount and distribution of centres of ossification of the skull of frog and mammal. There is no parasphenoid in the mammal*; and, instead, a complete series of ossifications, the median-, basi-, and pre-sphenoids, and the lateral ali- and orbito-sphenoids occur. The points can be rendered much more luminously in a diagram than in the text, and we would counsel the student to compare this very carefully with that of the Rabbit. * Faint vestigeal indications occur in the developing skulls of some insectivora. Section 34. -Cranium_ -Nasal_ (paired), -Vomer_ (paired) -Fronto-Parietal_, Sphenethmoid Bone (median), Eye, Pro-otic Bone, Otic Cartilage, Ex-occipital (paired) -Para-sphenoid Bone_ -Upper Jaw_ -Pre-Maxilla_ (paired), -Palatine_ (paired), Pterygoid (paired), -Squamosal_, Quadrate Cartilage {To 1.} -Maxilla_ 1. Quadrato-Jugal -Lower Jaw_ Mento-meckelian, -Dentary_, -Articulare- [-Angulo Splenial_] Section 35. -Points especially- [Additional points] to be noticed are: (1) The otic capsule (= periotic bone) of the dog ossifies from a number of centres, one of which is equivalent to the frog's prootic. (2) The several constituents of the lower jaw are not to be distinguished in the adult mammal. (3) The frog has no lachrymal bone. Section 36. We are now in a position to notice, without any danger of misconception, what is called the segmental theory of the skull. Older anatomists, working from adult structure only, conceived the idea that the brain-case of the mammal represented three inflated vertebrae. The most anterior had the pre-sphenoid for its body, the orbito-sphenoids for its neural processes, and the arch was completed above by the frontals (frontal segment). Similarly, the basi-sphenoids, ali-sphenoids, and parietals formed a second arch (parietal segment), and the ex-, basi-, and supra-occipitals a third (occipital segment). If this were correct, in the frog, which is a more primitive rendering of the vertebrate plan, we should find the vertebral characters more distinct. But, as a matter of fact, as the student will perceive, frontal segment, parietal segment, and occipital segment, can no longer be traced; and the mode of origin from trabeculae and para-chordals show very clearly the falsity of this view. The vertebrate cranium is entirely different in nature from vertebrae. The origin of the parietals and frontals as paired bones in membrane reinforces this conclusion.