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

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the braln wall were.due to marked reduction of ventricular volume(Robertson et
al .', 1967 ); the volume of brain tissue and the mitotic densíty were
actual ly reduced.. Neural overgrowth therefore cannot be the cause of
vIral-induced dysraphism. Similarly Bergguist (1960) found that over-
. growth of the chîck brain, produced by injury of the notochord wíth
removal of the fourth neurornere at Stages I1"I4, was assoc¡ated w¡th
lncreased mÌtosis but not increased neural volume.
Desplte Pattents description of overgrowth at early stêges, there
is no evidence that non-closure (myeloschîsis) is caused by íncreased
neural volume. ln the present series of chîck embryos,neural volume _dÌd
not lncrease durlng or after the establ ishment of myeloschlsis. lt
would be interesting to measure neural volumes in older embryos with
rryeloschîsis.
T¡s hypothesis that dysraphísm arises by ruÞture of the closed neural
tube due to hydromyelia (Gardner 1961 , 196\, 1972), was ba_sed on studies
by tJeed (1917¡ lgZZ; 1937-38) of the dynamics of cerebro-spinal fluid.
These experiments, however, relied on perfusion of the venticular system,
with consequent changes în hydrostatic pressure. As further evídence of
rupture of the rhombic roof due to hydromyelia, Padget (1968, l97o) quotes
studies by Bonnevie (1934) on the mouse mutênt later called myelencephalic
blebs.. Bonnevie belíeved that exencephaly and multiple congenîtál defects
assocîated with subepìdermal fluid blebs, were secondary to brain rupture
caused by hydronryelia. Reexamination of these mice by carter (1956, 1959) has
shown that exencephaly precedes bleb-formation, ând that th'e blebs (which
are responsible for some defects) are derived fron mesenchym¡l tissue fluíd.
Hydromyelia and rupture of the neural tube after closure does not
explaln the ex¡stence of dysraphism in human embryos of horizons lmmediately