1900. J 



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 the prominences which can be seen on the skull of the 

 summer calf a little behind the first rudiments of the horn-cores, on 

 the fronto-parietal suture, have something to do with the formation 

 of these exostoses l . At their highest degree of development their 

 appearance is such as is represented on Itichardson's plate hi. 

 (I.e.). As 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 repre- 

 senting 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 horn-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 

 P. 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. 



