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and trochlearis from the visceral branches of the trigeminus. 
“Similar views have been put forward by several authors. 
| feel most inclined to the following view. In the 
first chapter 1 made the supposition that the ventral spinal 
roots are to be derived from a similar diffuse plexus as 
„observed by FRAIPONT (1887, p. 36) in Polygordius, inner- 
vating the longitudinal musculature and directly connected 
to the epidermis. Such a plexus evidently owes its existence 
to the close application of the rudiment of the longitudinal 
musculature to the epidermis in the Annelid, or to the nervous 
tube in Chordates. Afterwards a concentration has caused 
the nerve fibres of the plexus, which has still a diffuse 
„character in Amphioxus, to unite into more distinct nerves. 
Thus evidently the close application of the myotomes to 
the epithelium of the medullary tube in young stages, 
has been the cause of the growing out of the cells of the 
latter into nerve fibres We cannot wonder then that also 
the praemandibular “somite’’, developing muscle fibres, 
has as a consequence acquired its own nerve. It will be 
evident from the above considerations, that to the ventral 
“Spinal nerves and tne eye-muscle nerves no primary value 
can be attributed in judging the metameric structure of the 
Vertebrate body. They play only a passive rôle. 
Thus after all we see that the arguments in favour of 
the view that the praemandibular cavities represent the first 
segment, are not so conclusive as they might appear at 
first sight. GEGENBAUR (1887. p. 103) states: “dieses 
Kopfmetamer ist etwas von allen Uebrigen Verschiedenes, 
da es eines ventralen Abschnittes entbehrt” and con- 
cludes: “das nach meiner Auffassung erste Metamer wird 
durch das zweite Somit, den ersten primitiven Kiemenbogen 
oder den Kieferbogen gebildet”, and to a similar conclusion 
HIS (1887, p. 446) was led in the same year: “dem vor- 
deren Kopfmetamer gehören eigentümlich zu: der Complex 
der Trigeminusganglien und von motorischen Nerven die Nn. 
-oculomotorius, trochlearis und die Portio minor Trigemini.” 
be rendered probable by the application of 
the principles of my theory, that part of the ectodermal 
investment of the oral cavity is derived from the prae- 
cerebral part of the apical plate, i.c. that part of the original 
apical plate of Annelids which is not infolded by the 
formation of the brain and lies accordingly in front of the 
transverse cerebral fold and the animal pole. Thus part of 
