1. No signs of internal structure can be seen in the human erythrocyte, even under the best conditions of dark‐ground illumination, using an objective of numerical aperture 1·32.2. There are two forms of crenation, due, apparently, to different causes.3. Cells after the Goughian change show an irregular granularity of the surface.4. Cells immediately before hæmolysis assume a spherical form.5. The remains of cells hæmolysed by saponin show the wrinkled appearance of collapsed bladders.The expenses of this research, which was carried out during the tenure of a Carnegie Research Scholarship, were defrayed by a grant from the Earl of Moray Fund.