284 
the manner of the dorsal nerves. As a consequence of this mode of 
origin, a considerable portion of the latero-ventral surface of the spinal 
cord is occupied by the exit of the motor nerves. They extend over 
about one third of the length of each segment and occupy also a 
considerable dorso-ventral space. Wherever the motor nerves arise 
the surface of the spinal cord shows a peculiar differentiation which 
has been described by RETZIUS!) in specimens prepared in the me- 
thylen-blue method, as a low long elevation of punctate substance in 
which the motor fibres apparently lose themselves. With gold-chloride 
this punctate elevation appears as a dark plate constantly marking 
the origin of the motor nerves and found nowhere else on the surface 
of the neural cord save at the points whence arise the 
fibres that pass to the no- 
tochord. Here also the occur- 
¢@ 9 
® e © . . 
RESTE rence of the peculiar deeply stained 
vp plate is constant, though it is small 
in size, and proportionate to the 
number of fibres passing to the 
openings in the chorda sheath 
(fig 3): 
Fig. 3. Sagittal section stained with gold-chloride. Oh. chorda. Ch.sh. chorda 
sheath. n.c. cells of central nervous system. Z.fd. longitudinal fibres of central nervous 
system. v.fb. fibres to notochord. v. pl. plate through which they pass. 
From these considerations it would appear that we have to do 
with fibres, not mesodermal but ectodermal in origin. If they are 
merely supportive in function, and analagous to the spongioblast fibres 
described by His, they offer an unparalleled exception in passing beyond 
the limits of the nervous system. If truly nervous, they constitute a 
third and ventral line of nerves peculiar to Amphioxus, unsegmental 
in origin, and of undetermined function save as their origin in ven- 
trally placed cells, their ventral exit from the spinal cord, and the 
peculiarly differentiated plate that marks their exit, ally them more 
closely to motor than to sensory nerves. 
My thanks are due to Professor WIEDERSHEIM in whose labora- 
tory this investigation was made, and to the Staff of the Zoolo- 
gical Station®at Naples, where I collected and fixed the material. 
Freiburg i. B., 3. Febr. 1892. 
1) G. Rerzıus, Biologische Untersuchungen, Neue Folge II. 
