ORIGIN AND STRUCTURE OF THE HEAD 89 



and trochlearis from the visceral branches of the trigeminus. 

 'Similar views have been put forward by several authors. 



1 feel most inclined to the following view. In the 

 first chapter i made the supposition that the ventral spinal 

 roots are to be derived from a similar diffuse plexus as 

 -observed by FRAIPONT (1887, p. 35) 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, 

 ihas been the cause of the growing out of the cells of the 

 Jatter 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 role. 



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 alien Uebrigen Verschiedenes, 

 da es eines ventralen Abschnittes entbehrf 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 gehoren eieentumlich zu: der Complex 

 der Trigeminusganglien und von motorischen Nerven die Nn. 

 •oculomotorius, trochlearis und die Portio minor Trigemini." 



It may 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 ceiebral fold and the animal pole. Thus part of 



