The following scheme of dissection has been found useful for studying the effects of teratogenic agents on rat fetuses. In our laboratory dissection is usually undertaken on the 21st day of pregnancy, one day prior to the usual time of parturition. - 44- I. Measurments 1) Measure the crown-rump distance 2) Measure the trans-umbilical distance H. General inspection 1) Check for cleft lip and palate 2) Check for exencephaly, anencephaly, or other head defect. 3) Inspect for ectopia cordis, exomphalos, ventral herniation or others. 4) Look for cranioschisis or spina bifida 5) Check digite and limb 6) The eyes 7) The tail should be inspected. m. Examination of Head Make three cut 1) At eye-level 2) Across the central part 3) Across the ears VI. Opening of Body Cavities. On raising the flap of the central abdominal wall the umbilical vein is stretched and must be cut. Check for persistant right umbilical vein. V. Examination of Thorax Observe any abnormalities of the lungs and pulmonary vessels. Abnormalities of many of the great vessels are common where teratogenic agents are employed. When cardiovascular abnormalities are encountered, it is usually advisable to stop dissection and compelte examination by sectioning the material in paraffin. \I. Examination of Abdomen and Pelvis. The rat has no gall bladder. Check the diaphragm for abnormal openings, and for possible abnormalities of the kidney. Hydro-ureter and hydronephrosis are easllyrecognlzed.