ORIGIN AND STRUCTURE OF THE HEAD 139 



it has been enfolded and has given rise to the praechordal 

 part of the brain. The eyes, formerly situated on the surface 

 of the prostomium, have been closed over at the same time 

 and, striving to regain the light, have given rise to the eyes 

 of the inverted type characteristic for Vertebrates. 



In three respects Petromyzon distinguishes itself from 

 all other Craniates and at the same time stands nearer to 

 Amphioxus: the segmentation of the mesoderm into somites 

 is complete (Hatschek, 1910), the dorsal and ventral spii.al 

 nerves remain separate (RANSOM and THOMPSON, 1886) and 

 the series of somites continues in a very distinct way in the 

 head as far as the prostomium. The four anterior somites, 

 being the trigeminus- or mandibular somite, the facialis- 

 acust cus- or hyoid-somite, the glossopharyngejs- and the 

 vagus-somite, in the beginning do not differ in any fundamental 

 respect from the subsequent ones. The first two may be 

 termed pro-otic, the latter two are post-otic; the auditory 

 vesicles belong to the second head-segment, as is the case 

 i<i all other Craniates and in the majority of the Annelids 

 provided with organs of equilibrium, just as in Amphioxus the 

 first or mandibular somite sends out a prolongation into 

 the prostomium, which afterwards sepaiates from it as the 

 "praemandibular somite" (HATSCHEK, 1910, p. 481), compa- 

 rable accordingly to the "head-prolongation" (Kopffortsatz) 

 of the first somite of Amphioxus. 



The first somite to be differentiated in ontogeny is in 

 this case not the mandibular somite, as in Amphioxus, but 

 the third, being the first post-otic one, and, as we shall see, 

 the first one to produce a permanent myotome (KOLTZOFF, 

 1902, p. 286, 318). The second and first are formed 

 somewhat later, in the same way as the fourth, fifth, etc. 



While in Amphioxus the endostyle arises as a ventral bulg- 

 ing out of the gut just in front of :he mouth, in the first somatic 

 segment, it originates in Ammocoetes, similarly to the rudi- 

 ment of the thyroid gland in Craniates, in the same segment but 

 consequently just behind the mouth (VAN Wyhe, 1907, p. 75) 



The ultimate fate of the anterior somites is somewhat dif- 

 ferent from that in Amphioxus where they all produce 

 regular myotomes. In Petromyzon this is only the case in 

 the post-otic somites while the two pro-otic ones, together 

 with the so-called "praemandibular somites", give rise to 

 the eye-muscles (KOLTZOFF, 1902, p. 343, 350,371). From 

 the post-otic somites a uniform and continuous series of 



