1. When the allantoic sac of the 4-day embryonic chick was grown as an organ culture in vitro, the endoderm showed great variation in its histological structure.

  2. The intact sac formed a closed vesicle lined by columnar epithelium containing many mucus-secreting goblet cells.

  3. When everted and grown in a stiff clot, the endoderm formed a tall columnar epithelium with few or no goblet cells.

  4. In a soft clot the everted sac became cystic and distended; the endoderm then resembled that of the normal chorio-allantoic membrane.

  5. In places the everted endoderm formed papillomatous, secreting outgrowths in both stiff and soft medium.

  6. It is suggested that these histological variations represent ‘modulations’ of the endoderm (Weiss).

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