| OCR Text |
Show 1900.] STRUCTURE OF THE MUSK-OX. 691 In the following period the development continues also in the same direction. The horn is lowered, the horn-core points more downwards, and the plug between " c " and " d " is formed. At the same time that these changes are going on in the distal and middle portions of the horn, the base of the horn-core is enlarged and expanded over a great part of the frontals and parietals, on which large exostoses are developed. It is possible, although not fully proved, that tbe prominences which can be seen on the skull of the summer calf a little behind the first rudiments of the horn-cores, ou the fronto-parietal suture, have something to do with the formation of these exostoses \ At their highest degree of development their appearance is such as is represented on Bichardson's plate iii. (I.e.). A s can be seen from that figure, they are highest, about 6 cm. or more, near the median line where they nearly meet from both sides. By-and-by the horny sheath encroaches in a median direction over these exostoses, and when it has come so far that it caps over them it cannot be driven out any more or be prolonged, because its shape hinders this. Fig. 1 B (p. 688) is a schematic figure representing such a stage. Fig. 3. Lateral view of the skull of an old male Musk-ox, showing the horn-core and the remains of the reabsorbed exostoses. The horn is, however, not yet fully formed although its length has reached its maximum. The continued growth tends to thicken the horny sheath, especially its upper layers. The bony substance 1 This is on the supposition that the exostoses are pre-formed independently of the liorn-cores, as seems at least partly to be the case in the Gnu. Cf. the interesting note on " The development of the horns of Catoblepas gnu " by F. E. Blaauw, Proc. Zool. Soc. London, 1889, Part I. On a skull of a young Musk-cow in the Zoological State Museum in Stockholm, there are also to be seen small exostoses quite independent of the horn-core and situated at a corresponding place. |
