The egg-shell is lined with two membranes each of which is composed of a network
of fibres several layers thick. 3IASSHOFF and STOLPMANX (196) studied fixed
material with the electronmicroscope and showed that each fibre consisted of a
central core with a fine, fibrillar structure surrounded by a fine-granular sheath.
Between the core and sheath there were gaps spanned by delicate strands. Fibres
from the inner and outer membrane were similar in structure, but the former were
thinner. Within both membranes some fibres were fused together in places by their
sheaths. These results were confirmed by SIMONS and WR·rz (zg6 3) but they found
that the gaps between core and sheath were not visible at all places. Only a few inner
membrane fibres were fused to outer membrane fibres at the junction between the
two membranes. A special layer, 2.7 g thick, on the inside of the inner membrane
was also described by Simo,B,s and WiERTZ (rg6 3). It had a fine-granular structure,
similar to that of a fibre sheath, and on its outer side it was fused with the sheaths
of adjoining fibres. While the inner and outer borders of this layer, in fixed preparations,
were of high electron-density the rest of the layer was perforated by a series
of openings arranged with varying degrees of regularity. The outer side of the membrane
is studded with mammillary cores that remain attached to the membrane when
it is removed from the shell with diaminoethanetetra-acetic acid at pH 8 or with
hydrochloric acid. Fibres from the outer membrane penetrate into mammillae of the
shell (Smriss, 195 8 ; MASSHOFF and STOr,rnzAw, 196) and SimoNs and WIERTZ
(1963) described how the structure of these fibres changes in the mammillae until
first the sheath and then the core fan out and eventually fuse with the matrix of the
mammillae so forming the mammillary core.