ORIGIN AND STRUCTURE OF THE HEAD 105 



the 4**^ {5^^) — 8^^ post-otic somites. As we shall see 

 below, they are partly intra- and partly post-cranial. 



Finally, in Amniotes the relation of the occipital myotomes 

 to the gill-slits has not been determined with equal exact- 

 ness as in Anamnia by those who adhere to the view that 

 both correspond. But, taking as an example VAN Bemmelen's 

 (1889, p. 254) instructive figure for Lacerta, we see that 

 here too five myotomes contribute to the formation of the 

 hypoglossus musculature, beginning with the first more or 

 less rudimentary myotome to be observed in the occipital 

 region. The most probable conception seems to me to be 

 that this belongs to the third post-otic somite (the second 

 being that of the primary va^ius). If this be right, this 

 somite is at the same time the last epibranchial one, since 

 the number of gill-pouches, being five, corresponds to that of 

 Amphibians. The hindmost of the five myotomes mentioned 

 above is post-cranial and corresponds to the atlas. According 

 to Corning (1895, p. 165», it is only from the 2"'' —5^'^ 

 of these myotomes that the hypoglossus-musculature in 

 Lacerta is derived, the first becoming rudimentary. In this case 

 no epibranchial myotome would participate iaits formation. 



Thus on the whole we may say that the hypoglossus 

 musculature originates from post-branchial myotomes and 

 that, if epibranchial myotomes contribute to its formation, 

 their number is yet not greater than one. Neal (1897, 

 p. 461) suggests that a temporary forward crowding of the 

 visceral clefts which causes the last epibranchial somite to 

 lie above and not in front of the last gill-cleft, as observed 

 by h mself in Acanthias and by Miss Platt (1898, p. 458) 

 in Necturus, may permit the ventral growth of this myotome 

 which would otherwise be prevented. Among the above 

 investigations, however, the only statement relating to the 

 participation of the last epibranchial myotome that has not 

 been opposed is that of Miss PLATT for NECTURUS, and here 

 the fact that the innervation is performed only by the first two 

 free spinal nerves seems not wholly to exclude doubt. 



The number of the myotomes producing the hypoglossus 

 musculature may vary and we get the impression that on 

 the whole it is greater in forms where also the number of 

 gill-slits under which this musculature will come to lie is 

 greater (Petromyzon), less in forms with less gill-slits (Am- 

 phibians, Amniotes). However, this is not a rule without 

 exceptions, since e. g. in rays, to judge from the number 



