24 somites, Johnson, Harvard Collection, Boston. Com- plete description, based on many models and histological study, published in monographic form by Johnson (1917). Rostral and caudal neuropores closed. Otic vesicle retains a narrow opening to surface. Three pharyngeal arches are present. Hepatic trabeculae invading framework of liver. Umbilical vesicle is compressed rostrocaudally, i.e., early um- bilical stalk. Probably originally 25 somites (Arey, 1938).
Harvard No. 714, 4-mm embryo. Described in detail by Bremer (1906). Probably belongs to stage 12 rather than stage 11 or stage 13. It shows some unusual features, such as arrested or delayed closure of the rostral neuropore.
Minot (Harvard No. 825) Stage 6b