Phenomena of ncurohiotaxis in the ojUic systcm 



29: 



ees, as formative factors in the sLructure of the eye, light being one of 

 the strongest tropistic factors that exist. 



Before discussing the formation ot the eye vesicle as a tropism, it will be 

 useful to consider the other organs of perception and examine the 01 dinary 

 nerves oi feeling, hearing and smelimg, concerning eventual pecuh'arities 

 pointing to phenomena of neurobiotaxis in their evohition and form. 



With regard to the ordinary nerves oí feeling there is but little to be 

 said. In human anatomy, the cells of origin of the spinal cord's poste- 

 rior roots have an extra-cerebral position in the intervertebral ganglion. 



The phylogenetic development of the hind-roots, however show s that 

 the extra-medullar position of their cells is a secundary acquisition. 



Fig. 2}¡. — Intraniedular spiíial >¿an<j;lii)n ctlls of the liintlroots in Am]jhioxus; 



after Jolmston. 



In the most primitive of vertebrates in Anip/iioxus, besides the sen- 

 sor}'' ganglioncells to be found in the sensory roots themselves, there are 

 spinal ganglioncells, lying in an ijitra-medullat'v position, as was demons- 

 trated by Johnston a. o. 



Figure 23 shows a great number ol the above named cells to be l\"ing m 

 the spinal cord, some of them seemip.g to penétrate into the root. Only 

 one third of these cells has left the spinal cord and is to be found in the 

 peripheral roots as bipolar cells. 



From this \ve may suppose that in this animal most of these cells 

 (as far as they are not derived from placode-cells M, origir^ally have an 



* Sensory ncuroepitlielial cells (a common featurr wilh invcrtehratesi, seem to 

 occur still in Amphioxus. In ln<^her animáis such epitelial ganglioncells oniy 

 occur in the placode-anlagen, the cells of which in furthcr development migrate 

 inward (stimulo concurrent), and form a part of the cronial ganglia. 



