1. “Insulin” content of microdissected islets was determined using the epididymal fat pad assay method. Measurements were made on fetal islets obtained from both normal and diabetic mothers and from normal postnatal rats up to five weeks of age.

2. The insulin content of the normal fetal islets, expressed as units “insulin” per gram dry weight of tissue, increases progressively with increasing fetal age; at birth the value approximates that of the adult.

3. The insulin content of the fetal islets obtained from diabetic mothers is between 7 and 19 per cent of the corresponding normal value.

4. In the acid-alcohol extracts of the pancreas of seventeen-day-old fetuses, small though measurable amounts of “insulin” were detected viz.: 0.23 units per gram wet weight of normal pancreas. There was a progressive increase in the extractable “insulin” content of the fetal pancreas with age; the “insulin” content of the twenty-one-day normal fetal pancreas was 5.3 units per gram of tissue. In the pancreas of the fetuses from diabetic mothers, the values were about one third of those in the normal.

5. Thus measurable amounts of insulin-like activity were found in the pancreatic tissue of seventeen-day-old fetuses; this is prior to the time that beta cell granules could be detected cytologically using the aldehyde fuchsin staining method.

6. The insulin content of the islet tissue remained relatively constant during the first five weeks of postnatal development.

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