The fine structure of the fibrous lamina on the inner aspect of the nuclear envelope in cells of various invertebrates is reviewed, and attention is drawn to the common occurrence of a similar but thinner layer in nuclei of cells in vertebrates. Certain superficial similarities are pointed out in the relations of fibrous lamina to the nuclear pores and of the basement lamina to the endothelial capillary pores. It is suggested that in future considerations of the physiological exchange of materials between nucleoplasm and cytoplasm the properties of the fibrous lamina must be taken into account.