Purpose: To observe the development of optic nerve, we examined four optic nerves from Siameses Twins by absolute counts of axons.Methods: Mean axon diameter, mean axon density, totally axonal population and optic nerve area were noted for each optic nerve. The mean axon diameter and the mean axon density were compared between paraxial(inner sectors) and cortical(outer sectors) areas of the nerves.Results: More myelinated axons were seen in the inner sectors as compared to the outer sectors (average 11 axons/1 000 jjum2 in inner sectors and 34 axons/1 000 u,m2 in outer sectors( P = 0. 036) . The myelinated fibers were also smaller(63 microns) in the outer sectors as compared to the inner sectors(72 microns) ( P - 0. 001). The average cross sectors area for the four 40 week stage optical nerves of Siamese Twins was 3. 32 x 103 as compared to 1 million axons for 32-week-old normals.Conclusion: Our finding of fewer axonal number and small myelinated fibers in the Siamese Twins suggests hypoplasia. Myelinat Objective: To observe the development of optic nerve, we examined four optic nerves from Siameses Twins by absolute counts of axons. Methods: Mean axon diameter, mean axon density, totally axonal population and optic nerve area were noted for each optic nerve. axon diameter and the mean axon density were compared between paraxial (inner sectors) and cortical (outer sectors) areas of the nerves. Results: More myelinated axons were seen in the inner sectors as compared to the outer sectors (average 11 axons / 1 000 jjum2 in inner sectors and 34 axons / 1 000 u, m2 in outer sectors (P = 0.036). The myelinated fibers were also smaller (63 microns) in the outer sectors as compared to the inner sectors (72 microns) - 0. 001). The average cross-sector area for the four 40-week stage optical nerves of Siamese Twins was 3. 32 x 103 as compared to 1 million axons for 32-week-old normals.Conclusion: Our finding of fewer axonal number and small myelinated fibers in the Siamese Twins suggests hy poplasia. Myelinat