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I t is not certain whether the regenerating process faithfully repeats the process of normal ontogenic differentiation. Hay (1957, 1958) described the mode of formation of endoplasmic reticulum in differentiating cartilage cells as being such that numerous small vesicles might coalesce to form new cisternae. ” But lately she has inclined to the opinion that the vesicles may originate from pre-existing elements in the Golgi region. Munger ( 1958a) studied normal morphogenesis in pancreatic acinar cells and stated that the ergastoplasmic cisternae seemed to merge into areas of cytoplasm containing closely packed tiny dense granules, and the membranous sacs might be formed by a condensation of particulate material.

ELECTRON MICROSCOPIC A N A L Y S I S OF SECRETION 21 According to Weiss (1953), the double membranes of the ergastoplasm are. flattened and pressed sacs. H e explained a concentric lamellae by postulating that many closed sacs were invaginated as though they were pushed with a finger and then cut in a plane perpendicular to the finger. To test the assumption that these structures were actually sacs, he tried the following experiments: he took pieces of fresh pancreas and put them in hypotonic solution, and found in electron micrographs that the flattened sacs became increasingly swollen with a rounded cross section.

KUROSUMI Bernhard and Rouiller (1956) observed the regenerating process of the ergastoplasm in rat -liver after a prolonged starvation, partial hepatectomy, and intoxication with carbon tetrachloride. The reappearance of ergastoplasm occurs under a close topographical relation to the niitochondria, and the membranes generally precede the appearance of the granules. They assumed that the membranes might be derived from invaginations of the cell surface as Palade (1956b) had suggested, and the granules from the nucleolus.