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of rapid ectodermal mitosis. However Gillette (1944) found a reduction
in the sl2e of. neural plate cêlls in Amblystoma during folding. Glaser ('l9.l4¡
19l6) sugsested that neuroepithel iar ceüs change from cuboidar to corumnar
and then to pyramidal shapes, due tc differential uptake of water at the
basal parts of the ce s. This was shown to be unrikely by the detect¡on
of only negliglble changes in the denslty of ,,uur"t plate cells during folding
ln Rana and Amblystoma (Brown et al., l94l), and by Gillettés finding of a
reduction ln cerr size. Derrick (1937) reported a hígher mitotic index in
neurectoderm than ín adjacent ectoderm of chick embryos during neururation,
but Bragg (1938) courd not detect a simirar differentiar mitosís in Bufo.
clllette (1944) and Holtfreter (1943) postulated a contractile surface
coat at the free ends of neuroepither iar ceus, responsibre for changes rn
cel I adhesion and shape.
ln a detai red analysis of neururation ín the bilaminar neurar prate of
xenopus, Schroeder (1970) described myotome erevation and epidermar expansíon,
as well as changes of shape in both layers of neuroepithel ial cells. Similar
changes have not been detected in the uniraminar neurâr prêtes of other
vertebrêtes, whích continue to show fording and crosure when cur tured rn ¡solatlon
(Roux, 1885; Boerema, 1g2Ð. The assertion by C. O. Jacobson (1962)
that neural elevation and folding in the Axolotl is produced
by forces generated in the underrying chorda-mesoderm has now been refuted by
Karfunkel and Burnslde independently (Karfunkel , 197Ð. Thus intracellular
mechanisms must be capable of generatlng the forces needed for neurar closure.
I'licrotubules running rn the long axis of neuroepither iar celrs have
been reported in embryos of Triturus (Waddington and perry, 1966¡ Burnside,