. Text-book of embryology. Embryology. XIII EOTIFEEA 425 dorsal side of the foot, a hollow invagination of ectoderm appears, this is the proctodaeum, that is the rudiment of the anus and hind-gut. â We have already seen that the hinder endodermal rudiment is contained in the foot, but before the anus is formed it begins to be displaced by a solid terminal invagination of ectoderm, which is the rudiment of the foot gland. In the following period of development the head region under- goes complicated developments. The proboscis is bent ventrally. Between it and the cuticular strips, mentioned ab
. Text-book of embryology. Embryology. XIII EOTIFEEA 425 dorsal side of the foot, a hollow invagination of ectoderm appears, this is the proctodaeum, that is the rudiment of the anus and hind-gut. â We have already seen that the hinder endodermal rudiment is contained in the foot, but before the anus is formed it begins to be displaced by a solid terminal invagination of ectoderm, which is the rudiment of the foot gland. In the following period of development the head region under- goes complicated developments. The proboscis is bent ventrally. Between it and the cuticular strips, mentioned above, a solid invagina-. eAd"" FiQ. 336.âOptical sagittal sections through two late embryos of Callidina russeola iu order to show the formation of organs. (After Zelinka.) A, stage of predominant growth of the tail, B, later stage: predominant growth of the head region, , foot gland ; cer^, cer^j first and second rudiments of the cerehral ganglion ; endX, endo- derraic rudiment of the gut; end^, endodermic rudiment of the excretory bladder; gon, genital rudi- ment ; prob, rudiment of proboscis : proct, proctodaeum ; sal, salivary gland; siomi, inner stomodaeum or mastax ; sttm?, outer stomodaeum; ten, tentacle; tr, border of region of apical plate which gives rise to the " wheels "; troph, rudiment of one of the trophi or jaws. tion of ectoderm appears on each side of the trochal disc. Then the whole head region is retracted and surrounded by an upgrowth of the ventral lip, which, however, does not quite reach the dorsal side. Later this upgrowth develops the outer cilia known as the cingulum. Still later, however, the whole head regionâproboscis, and apical shelf, is again protruded; and the cuticular strips are specially thickened at each side, where the shelf shows an indentation. The ectodermal thickenings between these and the proboscis are protruded as hemispherical bosses. Subsequently these are again invaginated, and remain so till the embr
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