In mitotic cells of Walker 256 carcinoma some four-layered lamellar structures were observed which had the appearance of paired cysternae of the ER. Two inner membranes were regular, smooth surfaced, and closely applied to each other. The two outer membranes were somewhat irregularly placed in relation to the inner pair; they showed attached RNP particles and connections with cysternae of the ER. The membranes often appeared to radiate from the region of the centrosphere towards the compact mass of chromosomes. Thus, they lay amid the spindle fibres and are referred to as "spindle lamellae." They approached the centrioles closely but were not observed to be continuous with them. They appeared to terminate in the pole of the spindle by joining smooth surfaced membranes in the centrosphere. Their equatorial termination was in relation to the chromosomes. At the surface of the chromosome mass they frequently split into two double membranes, which were closely applied to the chromosome substance. The most prominent and complicated membranes were seen in anaphase cells. An hypothesis is advanced which ascribes the development of the nuclear membrane to the spindle lamellae.

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