J 023 



In the Selaohii and, indeed, getierall}' in the Cianiota the first 9 

 myotonies belong to the head. It would now be interesting to ki»ow 

 under which myotome the ilio-colon ring of Amphioxus is situated 

 at its tirst appearance. This appearance takes place in the stage 

 with only one gill cleft and an open anus. 



Although in the course of time I have made hundreds of prepa- 

 rations of this stage, stained and imbedded in all sorts of ways, 

 I have not been able to count the number of myotomes. Their 

 boundaries, which are clearly discernible in earlier and later stages, 

 were not visible in this stage, not even in series of sections '). 



It is possible that these boundaries are to be seen in living larvae. 

 In any case Hatschek (J 881, fig. 64) indicates in a sketch of such 

 a larva that there are 20 myotomes present; the ilio-colon ring lias 

 however escaped his notice. If now the place of this ring, which is 

 very clear in my preparations, is compared with the sketch of 

 Hatschek, one comes to the conclusion that it must be situated 

 approximately under the 9''' myotome, thus at tiie end of the head 

 region. 



In other words: Not only the fore-gut (prosenteron) but also the 

 mid-gut (mesenteron) originally lies in the head region, and if this 

 is also the case in the Craniota, as may be expected, then it is no 

 wonder that also the mid-gut is supplied by a cranial nerve, tiie 

 n. vagus. The conception "head-gut" ought then not, as is at present 

 the case, to be synonymous to fore-gut, but must include the 

 mid-gut also. 



1) One could speak here of a cryptomelameric stage, a characteristic — amongst 

 others — that these larvae have in common with the Appendicularia and the 

 larvae of the Ascidians, in which the metamery is at present still denied by various 

 investigators. 



