MORPHOLOGY OF CRANIAL :\rrSCr>KS IN SOME VERTEBRATES. 275 



formation of the liypobrancliial spinal muscles and the 

 num])er of trunk-segments included in the skull (vide table, 

 p. 299). In general, the less the number of trunk- segments 

 included in the skull, the greater tendency there is for 

 the most anterior trunk myotomes to take part in the 

 formation of the hypobranchial spinal muscles, and the less 

 the number of anterior trunk myotomes atrophying -svithout 

 taking part in their formation. The table also shows that 

 the hypobranchial spinal muscles are derived from a variable 

 number of trunk myotomes. The number varies from two 

 (Rana) to five (Scyllium). Derivation from the smallest 

 number is probably the most primitive condition. The number 

 does not vary with that of the trunk-segments included in 

 the skull, nor with the number of head-segments. 



Lingual Muscles. 



The researches of Gegenbaur and Kallius in Salamandrina 

 have shown that the geuio-glossus is developed from the 

 genio-hyoideus and the steruo-glossus from the steruo- 

 liyoideus ; the former ends between the lingual glands which 

 form tlie fore part of the tongue, and is " also ein Driiseu 

 Muskel" (Gegenbaur) ; the latter becomes attached to a plate 

 of dense connective tissue which probably develops at the site 

 of fusion of the primitive tongue with the glandular portion. 

 Kallius has also shown that in the Anura the genio- and hyo- 

 glossus are developed from the genio-hyoid. The genio- 

 glossus grows towards the subsequently glandular, precopular 

 field, and later, the hyo-glossns fibres cross those of the 

 genio-glossus. 



To this may be added that in Rana the genio- and hyo- 

 glossus are developed from the inner division of the genio- 

 hyoid, which (vide p. 270) is proliferated from the inner side 

 of the primitive muscle. 



In 27 mm. larvae of Alytes the genio-glossus ends free 

 beneath the mucous membrane of the precopular field, and 

 the hyo-glossus passes forward at first below and then lateral 



