A Sex Difference in Chromosome Lengths I N

During the last two years we have had occasion to accurately draw and enlarge human chromosomes. Both male and female, embryonic and adult, tissues have been studied. In surveying the first or longest chromosomes of the series we were early struck by the greater length of this chromosome in the male. Five independent series of measurements have furnished invariable confirmation of the earlier observations. There are many pitfalls for the investigator who aims a t accuracy and comparable results with the mensuration of chromosomes. Prophase chromosomes are invariably slightly longer than those assembled a t the equatorial plate after dissolution of the nucleus; varying methods of fixation and dehydration shrink tissues unequally; and finally there are differences in the length and size of chromosomes in various cells especially in embryonic cells as compared with adult ones. I t is rather interesting that different types of cells vary somewhat in chromosome length as they undoubtedly do in the size of the cell itself. A striking instance of this is found in the lutein cells in the corpora lutea of the ovary a t the time of pregnancy. The cells are greatly enlarged and the chromosomes have a corresponding increase in size, the length and diameter being approximately two or more times those of the chromosomes of the surrounding cells. For all of these reasons we have sought to compare chromosome lengths only in the same phase of mitosis (late prophase) from the same type of tissue (mesenchyme) in embryos of approximately the same age, fixed and dehydrated by identical processes. Many human embryos of both sexes were explored for our work, but an immediate fixation (at the operating table) with resulting clarity of the chromosomes was secured in only two embryos of approximately the same age; a male of G. L. 19.5 to 20.5 mm and a female of G. L. 25 mm. In both cases we selected ten nuclei of the mesenchyme situated near the ectoderm, displaying clearly all of the forty-eight chromosomes, which were then individually drawn a t 3600 diameters. The original