Abstract
The zymogenic secretory cells of Hydra viridis are scattered between the digestive muscle cells of the gastric region. The mature zymogenic cells are located along the apical surface of the gastrodermal epithelium and contain numerous spherical secretory droplets. They appear to differentiate from stem cells located near the mesoglea. These stem cells resemble epidermal interstitial cells and are filled with free ribosomes. They differ from the interstitial cell in that they usually possess a small amount of granular endoplasmic reticulum. During the process of differentiation they elaborate a highly organized system of granular endoplasmic reticulum. This system becomes dispersed into vesicles as the secretory product is synthesized. There is no indication that the Golgi apparatus participates directly in the formation of the secretory droplets, and there is no indication of a membrane bounding the mature secretory droplet.
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