Summar>. The developmeni of the ^inc-dcficient rai ombryo has been \tudicd in vitro using embryo cullure techniques. Normal 9-5 day embryov culiured for 4H h in ^crum obtained from /inc-deficient rais gre«' and developed to the same extent 3s those cultured in /inc-replcte serum, binbryos from dams which had been fed a /inc-del'icient diet since mating were also removed for culturi:. Such /inc-det'icient cnibryns tell iiitu two broad tnorpholnmicjl caicgorics. One group appeared identical to the normal embryos, \Uiile the others had appareinly normal visceral yolk sacs but small embryonic poles and retarded or abnormal embryonic development. Cultureof Ihe first group in either ;inc-deficient or replete serum produced morphologically normal embryos; however, those which appeared abnormal at day 9-5 were grossly malformed after 48 h incubation in either sera. When embryos were cultured in the presence of "Zinc, Ihe mosl severely affected zinc-deficient embryos accumulated as much ^inc as the zinc-replete and apparently unaffected ?inc deficient embryos, indicating that the malformations do not arise from an inability of ihe embryo or yolk sac lo accumulate /me from the surrounding fluid. The results from these studies suggest that the tirratogenic effects of ?inc deficiency cannot be induced by direct culiure of rinc-replcte embryos in /inc-deficient serutn. Furthermore, it would appear that maternal /inc deficienc) can exert its tcratological influence ptior to day 9-5 of scsiaiion and ihai these effects are npi readily reversible.

The architecture of the basal region of the subcommissural organ (SCO) and the subjacent neuropil was studied in the brush-tailed possum, Trichosurus vulpecula (Marsupialia). Several structural features suggest that the basal mode of SCO-secretion may be as prominent as the well-established apical secretion. Some of the features that speak in favour of basal secretion are: (1) the existence of deep processes of secretion-laden SCO cells which reach and surround the capillaries in the hypendyma and the subjacent neuropil; (2) the presence of perivascular spaces, some of which may contain a material that resembles the secretory product of the SCO; (3) positive staining by means of paraldehyde-fuchsin of secretory material in the pericapillary zone of vessels in the hypendyma and its corresponding neuropil; (4) presence of labyrinths of the basal lamina of capillaries and associated nerves. The presence of nerve and other cell processes adjacent to the perivascular space, labyrinths and capillary wall suggests discharge into the capillaries of material such as the basal secretory product of the SCO. However, the absence of fenestrae in capillary endothelium in the context of the foregoing observations is enigmatic and speaks against the possibility of a conventional release of secretion. Nevertheless, it is possible that a secretory product containing particles of low molecular weight combined with some specific features of the local capillary endothelium, which facilitate transport, may be critical factors making possible transfer of such product(s) from the SCO to the capillaries.

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