Slide showing a five-part diagram of 'the pachytene relationships of the sex heterozygote the development of the differential segments of X and Y leading to the disappearance of Y'. Slide shows a five-part diagram of 'the pachytene relationships of the sex heterozygote the development of the differential segments of X and Y leading to the disappearance of Y'. Diagram parts are labelled 1 to 4, which have the title 'pachytene' and the fifth part is labelled 'metaphase'. This part is further labelled A2, B2, B3, C2, and C3. Title below diagram reads: 'Fig 115 - Diagram showing by the pachytene relationships of the sex heterozygote the development of the differential segments of X and Y leading to the disappearance of Y (4). The metaphase associations of certain types are given and depend on the position of the differential segments (black and white) in relation to the centromere'. Late 19th to early 20th century