This project was the first study of the genetics and embryo-pathology of exencephaly in a partially inbred mouse stock, SELH/Bc. Exencephaly was found in 17% of SELH fetuses. Analysis of day 8-9 gestation embryos indicated that SELH embryos were collectively normal in general development, but delayed in neural tube closure relative to overall or general development compared to two normal strains of mice, ICR/Be and SWV/Bc. Exencephaly was observed to be caused by a failure of fusion of the cranial neural folds in the mesencephalon region in SELH. All SELH embryos appeared to be abnormal in their pattern of cranial neural tube closure. They fail to make initial contact at the prosencephalon/mesencephalon junction region of the cranial neural folds (the first fusion in the cranial neural folds in normal embryos). SELH embryos, fused their anterior neural folds via an alternate (possibly passive) mechanism compared to normal strains of mice (SWV/Bc, and ICR/Be), by fusing the folds in a "zipper-like" fashion from the rostral base of the prosencephalon. This closure of the neural tube in genetically liable embryos by an abnormal sequence of events suggests a new model for anterior neural tube closure failure. Liability to exencephaly appeared to be fixed in the SELH stock. Of the 53 SELH males tested, all produced exencephaly. SELH animals were found to be heterogeneous in the frequency of exencephaly they produced, indicating that there are still genes segregating in the stock which affect the ability of embryos to complete anterior neural tube closure. Exencephaly in SELH does not appear to be caused by an autosomal dominant, sex-linked dominant or recessive, or simple autosomal recessive single gene, although F2, BCl, and BC2 exencephaly frequencies (after an outcross to ICR/Be) suggest that only a small number of genes are involved. A marked excess of female exencephalics was observed in SELH, F2, BCl, and BC2 fetuses. / Medicine, Faculty of / Medical Genetics, Department of / Graduate