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The emphasis of this review is on the primitive streak of the chick embryo, collated with such information as is available on the mouse embryo. Little modern work has been published on any reptile primitive streak. The following topics are considered: evolutionary significance; formation of the primitive streak; ingression and de-epithelialisation; the basal lamina; migration from the primitive streak of the endoderm and mesoderm; the role of the extracellular matrix; changes in cell adhesiveness; regression of the primitive streak and its role in body patterning; the primitive streak and induction.

A description of the fine structure of the yolk of the unincubated hen's egg has becn providcd, which will serve as a basis for further studies on yolk digestion. The gross components of the yolk (that is, free-floating lipid drops, ycllow and white yolk spheres together with their enclosed lipid subdroplcts, and aqueous protein fluid) could be recognized by phasc contrast and low power electron microscopy. The majority of the lipid drops, whether free floating or enclosed within yolk spheres, were composed of particles about 30 to 60 A in diameter. The protein component of thc yolk was found to consist of round profiles about 250 A in diameter. The surfaces of the yolk spheres were of three types, and it is difficult to decide which represents the true structure although reasons are given for believing that yolk spheres are not normally cncloscd by membranes identical with cell membranes.

The differentiation of the presumptive neural plate, the neural plate and the neural tube have been investigated in the chick embryo by SEM, TEM and histochemical techniques. The relationship of these tissues to neighbouring structures, including extracellular materials, has also been studied. When SEM micrographs of primitive streak stage embryos were examined in stereo, it was found that cells which had been invaginating at the time of fixation were similar in shape to fibroblasts migrating in vitro. It was concluded that SEM stereo pairs could provide evidence about the mode and direction of cell migration. Many more mid-bodies have been found associated with the developing neural tissue than with the lateral ectoderm. It was found possible to recognise mid-bodies not only by TEM but also by SEM. It is therefore proposed that SEM montages may be used for assessing which regions of a tissue have recently undergone extensive mitosis. The beads on the specialised threads seen in the early stages of development are now considered to be formed from mid-bodies. Similar, but unbeaded threads have been described which span the gap between the neural folds just prior to the dorsal closure of the neural tube and it seems probably that these threads help to close the neural tube. It is suggested that the beaded threads arise by incomplete separation of two daughter cells at mitosis, whereas the unbeaded threads form by outgrowth of cell processes.

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Although somites develop from the mesoderm in the tail of the chick embryo, they do not form to the tip of the tail. Previous work has shown that this terminal mesoderm possesses many of the characteristics of the segmental plate mesoderm which gives rise to the somites in the trunk. This investigation is aimed therefore at understanding why the terminal mesoderm fails to form somites. Mitotic and pyknotic rates have been obtained for the tail region of chick embryos between stages 13 and 27. Embryos were treated with colchicine, so that the mitoses were blocked in metaphase, and counts were made on serial sections. The overall mitotic rates were highest between stages 15 and 18. Regions of high mitotic rate, which are an indication of cell synchrony, were found in the tail bud mesoderm though not in a consistent location, and only infrequently near the anterior end of the tail segmental plate. In the trunk however (Stern and Bellairs 1984) a single peak of cell synchrony was routinely found near the cranial end of the segmental plate. It is concluded that the cells of the tail mesoderm are less synchronised in preparation for somitogenesis than are the corresponding mesoderm cells in the trunk. A further conclusion is that the tail bud is not per se a region of high proliferation, though there are patches of high mitotic rate. The overall pyknotic rate reached a maximum at stage 25; peaks of pyknosis corresponded initially with the mitotic peaks and were associated with the ventral ectodermal ridge and the tail gut. By stage 25 however, the high levels of cell death were restricted mainly to the tip of the tail.(ABSTRACT TRUNCATED AT 250 WORDS)