Scanning electron microscopic study of the fibrous rings of the arterial orifices in embryos of the 7(th) and 8(th) weeks.

The arterial fibrous rings were investigated in human embryos aged 7 and 8 weeks under light and scanning electron microscopes. In the present study it was shown that the arrangement of collagen and formation of fibres changes rapidly within one week. In the 7(th) week there are no collagen fibres and the netlike fibrils extend between processes of fibroblasts. At the end of the 8(th) week the collagen fibres are formed and they have different arrangements in the particular layers of the fibrous rings.

Early trabeculation and closure of the interventricular foramen in staged human embryos.

Internal differentiation of the ventricles was studied in staged serially sectioned human embryos of developmental stages 13-19 (postovulatory days 32-46). At stage 13 the trabeculation of both ventricles was advanced and the muscular part of the interventricular septum well marked. Dorsal and ventral endocardial cushions were fused and the atrioventricular canal was divided into two parts. In embryos at stage 18 the membranous interventricular septum was developing and the interventricular foramen was obliterated. At stage 19 the membranous part of the interventricular septum was becoming more cellular in structure.