front matter 1

Plate 21 is a survey of sections from seven levels

of the spinal cord in Y380-62, a specimen in the Yakovlev

Collection with a crown rump length of 56 mm (see Chap-

ter 5 in Altman and Bayer, 2001). All sections are shown

at the same scale. The boxes enclosing each section list:

the level, ranging from upper cervical to coccygeal; the

section number; and the total area of the section in square

millimeters (mm^2 ). Note that the areal measurements are

determined after fi xation, while the crown-rump length is

measured before fi xation. The unfi xed area of each section

could be 40 to 60% larger. Thus, the areal measurements

are given only for comparison purposes between levels.

Full-page normal contrast photographs of each specimen

are in Plates 22A-28A. Low contrast photographs with

superimposed labels and outlines of structural details are in

Plates 22B-28B.

The size differences between levels seen at GW8.5,

refl ecting a rostral-to-caudal gradient in spinal cord matu-

ration, persist in this specimen. The lumbar enlargement is

11% smaller than the cervical enlargement, and the small-

est cross-sectional area is at the sacral level. However,

the lower thoracic level is 37% smaller than the cervical

enlargement and 29% smaller than the lumbar enlarge-

ment, refl ecting regional size differences characteristic of

the adult spinal cord.

The dorsal neuroepithelium is no longer present in

this specimen. At all levels, the central canal is shaped like

a triangle, and is lined by a dense layer of cells, presum-

ably the ependymal layer. However, at the most caudal

level (Plate 28), a small ventral neuroepithelium may be

the source of glia. The lining of the persisting central canal

is considered to be composed of ependymal cells.

The gray matter is continuing to show maturational

changes indicative of the adult spinal cord. In the ventral

horn, the motoneurons are segregating into more discrete

motor columns than in the GW8.5 specimen. The thoracic

ventral horn has a large accumulation of motoneurons in

the medial motor column. At cervical levels, the interme-

diate gray has a distinguishable lateral cervical nucleus,

and an accumulation of large neurons near the central canal

that may represent the central cervical nucleus. However,

the migratory stream of large neurons that appeared to be

moving into the lateral cervical nucleus (seen in the GW8.5

specimen) is no longer evident. The dense groups of cells

that appear to be migrating to and settling in the substan-

tia gelatinosa, prominent at GW8.5, are less distinct in this

more mature specimen. In the white matter, all compo-

nents are expanding. The dorsal funiculus continues to fi ll

in with fi bers as it descends from the dorsal midline, fol-

lowing the retreating roof plate.

C. Spinal cord of a GW10.5 embryo

Part II: The First Trimester (concluded)