DEVELOPMENTAL ANALYSIS OF SCYPHOMEDUSAE

Summary The eggs of the scyphomedusae vary in size from 0–03 to 0–3 mm. diameter, a variation of a thousand‐fold in volume. Type of gastrulation is correlated fairly closely with egg size, the smallest eggs gastrulating by unipolar ingression, the largest by complete or partial invagination, while eggs of intermediate size gastrulate by a combined process of ingression and invagination. The nature of the subsequent development is equally correlated with egg size and type of gastrulation. The smallest become vermiform planula larvae incapable of constituting a polyp directly, the intermediate‐sized eggs form planulae which become polyps or scyphistomae, larger eggs may occasionally form actinula larvae and so form a polyp without having had an active planula existence, while the largest eggs develop directly into the ephyra‐medusa, omitting the scyphistoma stage entirely. In the case of the smallest, and of the smaller intermediate types, the blastula and planula stages have the capacity to grow as such without accompanying progress in differentiation. Budding may occur even at the planula stage. The scyphistoma develops four, sometimes six, gastric pouches, and forms tentacles from the rim of the oral disc, usually in the order 2, 4, 8, 12, 16, and by further addition of fours to attain sometimes as many as 32. It consists of a head and a stalk, though either part may be much the longer. Outgrowths from the body wall near the junction of head and stalk may form either pedal stolons for attachment, or buds. Buds may be liberated as free‐swimming pseudo‐planulae, may develop attached to the parent and then liberated, or may develop at the ends of long stolons. Buds of a statoblast function may be formed successively beneath the pedal disc. Tentacles of a fully grown scyphistoma may constrict off, move away as pseudo‐planulae and give rise to new polyps. There are regional differences in the capacity of pieces of the body wall to regenerate whole or partial polyps. Isolated endoderm has no capacity to reconstitute a polyp. Isolated ectoderm from any level can reconstitute a perfect whole. The type of strobilation depends upon the size and shape of the scyphistoma head or column. When the head is shallow and the stalk long, strobilation is usually monodisc, resulting in one, sometimes two ephyrae. When the head is large and more or less columnar, segmental constrictions appear, and strobilation is typically poly‐disc. Isolated segments of a polydisc strobila do not necessarily develop into ephyrae, but may reconstitute themselves into scyphistomae and become reattached as typical polyps. In polydisc forms at least, the initiation of strobilation depends upon the attainment of a large size, accumulation of food reserves, and a certain critical low temperature. Most ephyrae are liberated with eight lappets, but the first ephyra formed, that is, from the oral disc, usually has more than eight, while the last to be formed may have smaller numbers. The initial number, with their corresponding rhopalia, persists as the final number in the mature medusa.