Abstract
1. Ossification centres of sternum have been studied in 416 rat fetuses (176 in the prenatal and 240 in the perinatal period) after maternal administration of cyclophosphamide during 12th to 14th day of gestation. 2. Ossification defects of sterna were observed in 90 % of the treated fetuses and included missing of 1 or more sternebrae, their irregular shape, rudimentary size, bipartite sternebrae and abnormal intersternebral fusions, the latter showing 15 different patterns. 3. Absence of ossification centres for sternebrae was the most frequent anomaly. 4. Incidence of missing sternebrae was significantly reduced (P < 0.001) when gestation was prolonged (Barrow and Rowland, 1969) and fetuses collected in the perinatal period, indicating recovery from the suppressive effect of the drug on the developing mesenchyme. 5. Treatment on thirteenth day of gestation was found to be most teratogenic for inducing sternal anomalies. 6. Fifth sternebra was most commonly affected.
