_wr_, Wolffian ridge. _wt_, Wolffian tubules. _y_, yolk. EXPLANATION OF FIGURES 1-26 ON PLATES VI.-XXVIII. All of the figures, with the exception of the photographs and those copied by permission from S. F. Clarke, were drawn under a _camera lucida_. The magnification of each figure, except those from Clarke, is indicated below. The photographs were made by the author, and were enlarged for reproduction by the photographic department of the Smithsonian Institution. The other surface views were made, under the author’s direction, by Miss C. M. Reese. With the exception of Stage III., all of the figures of any one stage are given the same number, followed where necessary by a distinguishing letter, so that it is possible to tell at a glance which section and surface views belong together. The transverse sections are all cut in series from anterior to posterior. FIGURE 1. Surface view of egg. × ²⁄₃. 1_a_. Egg with part of the shell removed to show the chalky band in the shell membrane. × ²⁄₃. FIGURES 2 and 2_a_. Dorsal and ventral views respectively of the blastoderm before the formation of the notochord, medullary folds, etc. After Clarke. 2_b_-2_f_. Transverse sections of an embryo of the age represented in Figures 2 and 2_a_. × 43. 3 and 3_a_. Ventral and dorsal views respectively of an embryo a few days older than that represented in Figures 2 and 2_a_. After Clarke. 3_b_-3_m_. Transverse sections of an embryo of the age shown in Figures 3 and 3_a_. × 43. FIGURES 3_n_ and 3_o_. Two sagittal sections of an embryo of the same stage as Figures 3 and 3_a_. × 43. 4 and 4_a_. Dorsal and ventral views respectively of a slightly older embryo than the one shown in Figures 3 and 3_a_. Figure 4_a_ shows only the head region. After Clarke. 5 and 5_a_. Dorsal and ventral views respectively of an embryo of almost the same age as the preceding, to show the further development of the medullary folds. After Clarke. FIGURE 6. Dorsal view of an embryo only a day or two older than the preceding. After Clarke. FIGURES 6_a_-6_i_. A series of transverse sections of this stage. × 43. FIGURES 7_a_-7_h_. A series of transverse sections of an embryo slightly older than the one shown in Figures 4-6. × 43. (No surface view of this stage is figured.) 8 and 8_a_. Dorsal and ventral views respectively of an embryo with five pairs of mesoblastic somites. × 20. (Drawn by transmitted light.) 8_b_ and 8_c_. Two sagittal sections of an embryo of this stage. × 43. FIGURES 8_d_-8_j_. A series of transverse sections of the embryo represented in Figures 8 and 8_a_. × 43. 9_a_-9_m_. A series of transverse sections of an embryo somewhat more advanced in development than the one represented in the last series. × 43. FIGURES 10 and 10_a_. Dorsal and ventral views respectively of an embryo with eight pairs of mesoblastic somites. × 20. (Drawn chiefly by transmitted light.)