Plate 29 is a survey of sections from Y68-65, aspecimen in the Yakovlev Collection with a crown-rump
length of 108 mm (see Chapters 5 and 6 of Altman and
Bayer, 2001). All sections are shown at the same scale.
The boxes enclosing each section list: the level from upper
cervical to sacral/coccygeal; the section number from the
set of slides containing all the sections of that specimen;
and the total area (post-fi xation) of the section in square
millimeters (mm^2 ). Full-page normal contrast photographs
of each specimen are in Plates 30A-37A. Low contrast
photographs with superimposed labels and outlines of sec-
tion details are on facing pages in Plates 30B-37B.
The outer surface of this specimen has sharp spike-like projections, and the white matter is thin around the
gray matter, interpreted to be a shrinkage artifact of histo-
logical processing. Cells in the gray matter are not distinct,
except for the prominent clumps of large motoneurons in
the ventral horn, possibly due to inadequate penetration of
fi xative. In spite of these histological artifacts, this speci-
men is illustrated becaue it is the only one in the Yakovlev
Collection where sections of the spinal cord are consecu-
tively numbered from cervical to coccygeal levels.Most of the size differences between levels at
GW14 are similar to those in the adult spinal cord. The
smallest cross-sectional area is in the middle thoracic level,
which is 27% smaller than the sacral level. That the cervi-
cal enlargement level is smaller than both the upper cervical
and lumbar enlargement levels refl ects variability unique
to this specimen. Within the gray matter, the most obvi-
ous sign of ongoing maturation is the prominent columnar
arrangement of motoneurons in the ventral horn. Not only
are these columns larger than at GW10.5, but there are
more of them. Within the white matter, the dorsal funicu-
lus is deepening further in the dorsal midline, accompany-
ing the retreating roof plate.III. The Second Trimester