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

t68
with lack of contact in severar embryos showing myeroschisis. somite
developmènt aþpeared to be normal, with no cysts or hemorrhages.
0f the Stage 16 experlmental embryos several showed normal neural
closure. Several others showed rnyeloschisis at the lower somlte, protosom¡te,
êrid unsegmented mesoderm areas, giving way to a closed neural tube
(showing accessory canars and so derived from tair-bud materia¡) in the
caudal reg ion.
Two of the Stage t6 embryos however (lOf 35, 3OE 76) showed a different
type of neural defect in the lower somîte, protosomite, unsegmented meso-.
derm, and caudal areas. ln these embryos the neural tissue formed a
V-shaped or U-shaped mass, open on the dorsal aspect. The total volume
of neural tissue at the site of each lesion (but not at more cranîal levels)
was reduced when compared to Stage 16 contror or normar experimentar embryos.
0n fol lowing the canal of the neural tube down from the somite region
în these two embryos' it courd not be trêced con.tinuous¡y into the dorsar
half of the lesions. The neurar prate materiar was progressivery reduced
at the cran¡al end of each lesion, wh¡ch thus appeared to be composed
entlrely of ta¡l-bud marerial.(F¡gs. 77-82).
An absence of accessory canals, in contrast to the multiple canals of
Stage 16 control and normal experímental embryos, suggested an early
maturat¡on of the tail-bud material in the two resions. The defects were
covered by ectoderm above end berow and open in the mîddre section, though
not so smoothly contínuous with ectoderm as in the examples of myeloschisîs.
ln the caudal region, each les,ic¡n gave wây to a ctosed neural tube covered
by ectoderm and closely resembr in9 the caudar tube of Stage 16 control and
normal experimental embryos, though slightly reduced ín size.