_--- ---- --- _--,_-.-- --.-.

animals. With the Fink-Heirner stl;dy, ths? degenerated fibre tract md kminals appeared much coarser and easily d&&able in older than younger animal~a. This is to be &per+kd as Rager 1193 hi,s shown that the main par+_ of the iyelination proc~ takes place between 14 md 80 days after hat&ing,

In all aniruals receiving early unllater& erXlc1eatiol-J nr3 optic newe wa# 5f)tin bon%ering the optic tectum contralatir~,J ts the eye enucleation. Also f‘tintra- lateral to the enucleation, the stratum opticum was harclly recognizable, a& below the pial surface bu% above !p+mina IIg on]!: 3 layers can be rec~~mized corresponding tc> the appearance ulf tti optic ti “w? after early xemcwd of the optic vesicle describd by Kelly and Cowa [X3, r’olluwing uniia+xral er!~lea- tian the retinofugal fibers from the remaining eye revealed by both autoradio- graphy and degeneraGon &tidies showed only cosatralateral projection to the optic tectum and other r&y cent~s described ag ,-w, No anomalom~ fiber projectiuns were de&M zuiywhere else.

The lack of sprouting from Q,he remaining eye a&r ;1n8a,teral enucieatiorl might be expected as tha? visual sys%m in the chick at or shartly before the- trrrre of hatching wm found to be functiona&y quit2 m;).turt: [ 171 and GO new

morphological features were detectable in the re~inotectal pmojecticn [ I3 1, Tht7

maturity of the visual system @f the chick is attested by the present study which showed that after enucleatlon at hatching the degeneratiq optic: caxan~ were still visible even at 27 days poat-eraucleat~on. The long-lasting degenstratIon aqyrophilia ham been correl&d with m,riturity of the system undar study [ 14 1.

Mattif;y may be rune fztir discoumging sprouting in mm ;jrs&ma m h-m been demonstrated by preGous workers, for example, SEerr [l2] ) Zeong [ 151 and

Lund and Lund [16 J . In other sysbzms maturiky does not wern to be BYI obc.ta& lo sprouttig (e.g. ref. 9). Some other factors like sp~ies dlfferenrc [ &IO] arrd certai.p1 events occurring during embrytinic development may be factors preventtig sprouting from specific regions. Thus rfi I;hI: +iclr. vlstisll system, previous &lady 161 has show-r1 ;,,,.,& the r~6,ino-wecM con :! &ions in the? embryo stage are topographically orgmized zmd such osgmi,sar.~m is fixtX!l between the third aM fourth day of incubation. Any attimpt [~a f to alter swh

specificity of r&ino-txtaE pru)e&ion by p&id ablatior! of khtt ,y~ after tl- is petii,od has not resulti’d in any slomalou~ fibse forma?;iors from1 t;:if+ remaining part of the clp~ated eye.

REFERENCES