During stage B the medullary groove is shallow posteriorly, deeper in the middle part, and flattened out again at the extreme anterior end of the embryo. (Pl. 7, fig. 10_a_, _b_, _c_.) A similar condition obtains in the stage between B and C, but the canal has now in part become deeper. Anteriorly no trace of it is to be seen. In stage C it exhibits the same general features. (Pl. 10, fig. 2_a_, 2_b_, 2_c_.) By stage D we find important modifications of the canal. It is still shallow behind and deep in the dorsal region, Pl. 10, figs. 3_d_, 3_e_, 3_f_; but the anterior flattened area in the last stage has grown into a round flat plate which may be called the cephalic plate, Pl. 8, D and Pl. 10, figs. 3_a_, 3_b_, 3_c_. This plate becomes converted into the brain. Its size and form give it a peculiar appearance, but the most remarkable feature about it is the ventral curvature of its edges. Its edges do not, as might be expected, bend dorsalwards towards each other, but become sharply bent in a ventral direction. This feature is for the first time apparent at this stage, but becomes more conspicuous during the succeeding ones, and attains its maximum in stage F (Pl. 10, fig. 5), in which it might almost be supposed that the edges of the cephalic plate were about to grow downwards and meet on the ventral side of the embryo. In the stages subsequent to D the posterior part of the canal deepens much more rapidly than the rest (vide Pl. 10, fig. 4, taken from the posterior end of an embryo but slightly younger than F), and the medullary folds unite and convert the posterior end of the medullary groove into a closed canal (Pl. 8, fig. F), while the groove is still widely open elsewhere[180]. The medullary canal does not end blindly behind, but simply forms a tube not closed at either extremity. The importance of this fact will appear later. Footnote 180: Vide Preliminary Account, etc. _Q. Jl. Micros. Science_, Oct. 1874, Pl. 14, 8_a_. [This Edition, No. V. Pl. 3, 8_a_.] This and the other section from the same embryo (stage F) may be referred to. I have not thought it worth while repeating them here. In a stage but slightly subsequent to F nearly the whole of the medullary canal becomes formed. This occurs in the usual way by the junction and coalescence of the medullary folds. In the course of the closing of the medullary groove the edges of the cephalic plate lose their ventral curvature and become bent up in the normal manner (vide Pl. 10, fig. 6, a section taken through the posterior part of the cephalic plate), and the enlarged plate merely serves to enclose a dilated cephalic portion of the medullary canal. The closing of the medullary canal takes place earlier in the head and neck than in the back. The anterior end of the canal becomes closed and does not remain open like the posterior end.