NECTURUS 17 



distinct parts, cartilaginous and osseous, are to be recognized. 

 In the dry skull the cartilaginous parts are not readily made out, 

 but by soaking in water they usually become visible.* 



The cranium is at first developed in cartilage forming a chondrocranium, 

 which, while incomplete in parts, can be compared to that of the elas- 

 mobranch. As development proceeds this cartilage is in part con- 

 verted into bone (cartilage bone) and in part covered by other bones 

 (membrane bones) which never pass through a cartilage stage but are 

 developed from the deeper layer of the skin. The distinctions between 

 these two kinds of bone cannot be recognized in the adult, but only by 

 tracing the history. A list of each group occurring in the cranium of 

 Necturus is given here. 



Cartilage Bones: 2 exoccipitals, 2 epiotics, 2 prootics, 2 pterygoids, 

 2 stapes, and 2 columellas. 



Membrane Bones: 2 parietals, 2 frontals, 2 premaxillaries, 2 vomers, 

 2 squamosals, and 1 parasphenoid. 



Quadrates, pterygoids, and premaxillaries rightly belong to the 

 visceral skeleton, but they are so intimately united to the cranium that 

 they may be considered with it. 



In studying the skull, after recognizing the position of any 

 element, carefully trace its boundaries with the hand-lens, noting 

 the exact position of the joints (sutures) between it and the 

 adjacent bones. 



The cranium presents a base, where it connects with the trunk, 

 and dorsal and ventral surfaces. In the base is a large opening, 

 the foramen magnum, through which the brain connects with the 

 spinal cord. On either side of this foramen is an exoccipital 

 bone, which bears on its posterior surface an occipital condyle 

 for articulation with the atlas. 



On the side the exoccipital articulates with an opisthotic 

 bone which extends to both dorsal and ventral surfaces, while 

 laterally it forms the postero-lateral angle of the skull. 



Draw the cranium from behind, X4. 



The dorsal surface of the skull is covered behind by a pair 

 of large parietal bones which meet in the middle line, while in 

 front they articulate with a pair of frontal bones which extend 

 forward nearly to the tip of the snout. The tip of the cranium 



* It is well to keep the skulls in 95% alcohol in which a little Lyons blue 

 is dissolved. When required for study they may be placed in water which 

 swells the cartilages while the blue has stained these parts so that they are 

 distinctly seen. At the close of the exercise return them to the alcohol. 



