Figs. 1_a_, 1_b_, 1_c_. Three sections from the same embryo belonging to a stage intermediate between B and C, of which fig. 1_a_ is the most anterior. × 96 diameters. The sections illustrate (1) The different characters of the medullary groove in the different regions of the embryo. (2) The structure of the coalesced lateral and vertebral plates. (3) The mode of formation of the notochord as a thickening of the hypoblast (_ch´_), which eventually becomes separated from the hypoblast as an elliptical rod (1_a_, _ch_). Fig. 2. Section through the anterior part of an embryo belonging to stage C. The section is mainly intended to illustrate the formation of the ventral wall of the alimentary canal from cells formed around the nuclei of the yolk. It also shews the shallowness of the medullary groove in the anterior part of the body. Figs. 2_a_, 2_b_, 2_c_. Three sections from the same embryo as fig. 2. Fig. 2_a_ is the most anterior of the three sections and is taken through a point shortly in front of fig. 2. The figures illustrate the general features of an embryo of stage C, more especially the complete closing of the alimentary canal in front and the triangular section which it there presents. Fig. 3. Section through the posterior part of an embryo belonging to stage D. × 86 diameters. It shews the general features of the layers during the stage, more especially the differentiation of somatic and splanchnic layers of the mesoblast. Figs. 3_a_, 3_b_, 3_c_, 3_d_, 3_e_, 3_f_. Sections of the same embryo as fig. 3 (× 60 diameters). Fig. 3 belongs to part of the embryo intermediate between figs. 3_e_ and 3_f_. The sections shew the features of various parts of the embryo. Figs. 3_a_, 3_b_ and 3_c_ belong to the head, and special attention should be paid to the presence of a cavity in the mesoblast in 3_b_ and to the ventral curvature of the medullary folds. Fig. 3_d_ belongs to the neck, fig. 3_e_ to the back, and fig. 3_f_ to the tail. Fig. 4. Section through the region of the tail at the commencement of stage F. × 60 diameters. The section shews the character of the tail-swellings and the commencing closure of the medullary groove. Fig. 5. Transverse section through the anterior part of the head of an embryo belonging to stage F (× 60 diameters). It shews (1) the ventral curvature of the medullary folds next the head. (2) The absence of mesoblast in the anterior part of the head. _hy_ points to the extreme front end of the alimentary canal. Fig. 6. Section through the head of an embryo at a stage intermediate between F and G. × 86 diameters. It shews the manner in which the medullary folds of the head unite to form the medullary canal. Fig. 7. Longitudinal and vertical section through the tail of an embryo belonging to stage G. It shews the direct communication which exists between the neural and alimentary canals.