[Illustration: FIG. 82.—Stages in the segmentation of Amphioxus. A represents the eight-celled stage; B, the sixteen-celled; D, vertical section of C; F, vertical section of the blastosphere or blastula stage (E). (From Korschelt and Heider, after Hatschek.)] The egg is small (0.105 mm. in diameter when shed) and contains very little food-yolk. Segmentation is complete (Fig. 82, A), is nearly regular, and results in the formation of a hollow blastosphere (Fig. 82, E, F), the wall of which is one cell thick. The lower cells (Fig. 82, B, C, D) are slightly larger than the upper. Invagination of the lower cells then takes place (Fig. 83, A), resulting in the suppression of the blastocoele or segmentation cavity and the formation of an archenteron, at first shallow and opening widely to the exterior (Fig. 83, B), and then deeper and with the opening narrowed to a small posterior blastopore (Fig. 83, C). This "gastrula" stage differs from the blastosphere in having a mouth or blastopore, and in being two cell-layers thick—epiblast (ectoderm) on the outside and hypoblast (endoderm) within. It soon shows the future aspects of the body by its {131}elongation and shape (Fig. 83, C), as the dorsal surface becomes flat and the ventral convex, while the blastopore is at the posterior end of the dorsal surface. The blastopore soon closes, and the mouth and anus are formed independently later. [Illustration: FIG. 83.—Three stages in the formation of the gastrula of Amphioxus. In A the nuclei of the endoderm have been omitted; C has the dorsal surface uppermost, and the posterior end to the right (From Korschelt and Heider, after Hatschek.)] The epiblast cells become ciliated all over the surface, so that the embryo rotates within the thin covering which still surrounds it. And now all the chief systems of the body begin to be marked out. The tubular nervous system develops from a depression of the epiblast (the medullary plate) in the middle line of the flattened dorsal surface (Fig. 84, A, _mp_). The edges of the depressed area grow inwards and unite over the deeper layer of epiblast, which becomes the wall of the neural canal or embryonic nervous system (Fig. 84, D, _n_); and further back these edges of the medullary plate join one another behind the blastopore, so that the latter comes to open into the floor of the neural canal, thus forming the neurenteric canal (Fig. 85, A, _cn_). Anteriorly the neural canal (_n_) opens to the exterior for some time by the neuropore.