The arterial ring has grown much larger and the portion of the yolk where no vessels exist is very small (_x_). The brush-like termination of the venous trunk is still to be noticed. The two main trunks (arterial and venous) in reality are in close contact as in fig. 5, and enter the somatic stalk close together. The letter _a_ which points to the venous (blue) trunk should be _v_ and not _a_. Fig. 5. Circulation of the yolk sac of a still older embryo, in which the arterial circle has ceased to exist, owing to the space outside it having become smaller and smaller and finally vanished. EXPLANATION OF PLATE 10. (X. p. 298.) COMPLETE LIST OF REFERENCE LETTERS. _al._ Alimentary canal. _ch._ Chorda dorsalis or notochord. _ch´._ Ridge of hypoblast, which will become separated off as the notochord. _ep._ Epiblast. _hy._ Hypoblast. _lp._ Coalesced lateral and vertebral plate of mesoblast. _mg._ Medullary groove. _n._ Nucleus of yolk. _na._ Cells formed around the nuclei of the yolk to enter into the ventral wall of the alimentary canal. _nc._ Neural or medullary canal. _pv._ Protovertebra. _so._ Somatopleure. _sp._ Splanchnopleure. _ts._ Mesoblast of tail-swelling. _yk._ Yolk-spherules. 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_.