73 
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 spinal 
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 glossopharynge.s- and the 
vagus-somite, in the beginning do not differ in any fundamental 
tespect 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 
in 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-proloagation” (Kopfífortsatz) 
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 frst 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 the 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 
