OCR Text |
Show 8 DR. J. F. GEMMILL ON [May 12, posterior fontanelle is much larger than in a normal case (PI. I. fig. 4, / 3 ) . Each nasal cartilage is connected with a pair of palato-quadrate bars, the inner or adjacent elements of which converge, fuse, and end abruptly without being attached _ to a suspensorium; the outer bars are continued backwards on either side, and articulate with the hyomandibulars attached to the auditory capsules. Articulating with the united part of the inner palato-quadrate bars is a small twisted piece of cartilage, which passes downwards in the septum between the two mouth-openings, and represents an inner or adjacent pan of Meckelian cartilages. Dorsally, each nasal capsule is continuous with a pair of supraorbital bars, of which the outer elements pass backwards on either side to join the auditory capsules, while the inner or adjacent elements are connected with each other and with the outer bars over the pineal body. Behind this the adjacent bars disappear, the result being that over the region of the cerebral lobes there are two small anterior fontanelles, while over the optic lobes there is a single large middle fontanelle (PI. I. fig. 4). The hyoid bars and the branchial cartilages are normal, except that they diverge rather more widely than is usual. Each arch, however, may be looked upon as being composite, i. e. as consisting of the outer elements of a double set of arches. The suspensoria on either side of the two periotic capsules are normal. As mentioned above, the notochord is double in front, two notochords being present as far back as the fourteenth body-segment. These converge together at an acute angle and finally unite. The condition of the neural and harmal arch cartilages is illustrated in PL I. fig. 8. Where the two notochords are some little distance apart, the inner or adjacent neural arches are displaced so as to form a floor for the transversely-expanded spinal cord, while the inner harmal arches remain as small nodules of cartilage. As the notochords come closer together the inner neural and harmal arches alike disappear, while the outer arches assume a normal form and position. The brain and its cavities show a degree of duplicity which is indicated diagrammatically in PI. II. fig. 15. There are two pairs of cerebral lobes and two thalamencephala, diverging forwards from a single composite optic lobe region. The cerebral lobes and thalamencephala, besides diverging, are rotated slightly in such a way that they lie closer together dorsally than ventrally. There are two pairs of olfactory nerves, two pairs of optic nerves, and two sets of pineal diverticula. There are also two infundibula, which converge as they pass downwards and backwards. Each ends in a hypophysis sac after giving off the usual diverticula for the hypoaria (PI. II. fig. 17). The inner hypoarium on either side, owing to want of space, is smaller than normal, and lies above and in front of the outer hypoarium. The optic lobes show a remarkable transition between the double and the single condition. Their cavity and roof-parts are single, while the basal structures are doubled. There are thus two pairs of 3rd nerves. |