File:HarrisonJeffcoate1953 plate02.jpg

Plate 2

(All figures are at a magnification of x 180.)

Fig. 1. The embryo at a more posterior level (section 32) showing the diminution in width of disc endoderm. In this, and the next four succeeding figures, the transition from the taH columnar cells of disc ectoderm to the flattened amniotic ectodermal cells is clearly marked.

Fig. 2. Photomicrograph of section 34 showing the increased number of extraembryonic mesoderm cells at the lateral margins of the yolk-sac. The ectodermal cells in the centre of the disc are slightly irregular, simulating the formation of neural folds.

Fig. 3. The narrowing of the yolk-sac in its transverse diameter as seen in section 35. There is also an indication of the division of the yolk-sac into yolk-sac proper, and the allanto-enteric diverticulum (in the lower part of the endodermal cavity).

Fig. 4. Photomicrograph of section 36 to show the allanto-enteric diverticulum (a.e.d. in the figure) embedded in connecting stalk mesoderm.

Fig. 5. The region of the embryo (section 40) in which the disc ectoderm cells are clearly dividing (the primitive streak; 12.3. in figure) to contribute cells to the connecting stalk. A mitotic figure can be seen in this region.

Fig. 6. Section 42 of the embryo at the posterior end of the primitive streak in which there is no clear distinction between ectodermal cells and cells of the connecting stalk. No endodermal tissue is present at this level, and only the amniotic cavity can be seen. The part of the connecting stalk between embryo and chorion is seen in the top right-hand corner of the figure.

Reference

Lewis BV. and Harrison RG. A presomite human embryo showing an early stage of the primitive streak. (1953) J Anat. 87(2):124-9. PMID: 13044724