Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

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ectoderm and neuraì tlssue ln myelodysplasia.
The s.uggéstion that caudal levels of the neural tube, formed after
closure of the posterlor neuropore, are derived from the tail-bud was
flrst made by Biaun (1882), who described a caudal mass of cells undergolng
cavltation in avian enhryos. Development of the terminal part of
the hur¡an spinal cord from the tail-buci was described by Keíbei and Elze
(1908). Schumacher (1927) showed that multiple cavìtation of caudal neural
tlssue ls a normal process in the chick embryo.
ln birds, Hensents node increases ln size and lncorporates the
remalnder of the primlt¡ve streak, to form thê ta¡ l-bud at the 18-22
somlte stage ($eevers, 1!J2). Wetzel (1929) regarded the actív¡ty of the
tall-bud ês the same as that of the streak and node of earller stages, but
Hunt (193.l) and Seevers (1932) showed that it has ìost the capacity for
primary induction, and ¡s restr¡cted to the formatlon of posteríor trunk
structures.
Criley (1969) demonstrated an overlap zone în the caudal region
of the chlck neural tube, between material derived from the neural plate
(lying dorsalty) and material derîved from the tall-bud (lying ventrally).
This overlap zone (of 192-280 microns) was detectable between Stages I1
and 18, with fìsion occurring maximal ly at Stages 13-15. The taîl-bud
materlal extended up to at least somite 35, and the neural plate material
down to at least somíte 33. Removal of the most caudal section of the
neural plate showed that an appârently normal segment of spÍnal cord can
develop from the tail-bud ín complete isolatlon from the neural plate.
ln the present embryos the extent of the overlap was revealed by
qulte subtle changes in outl ine of neural tlssue at. caudal levels. ldentifylng
features from cranial to caudal were: fallure of the neural canal to