clear, but probably shews that the greatest growth is taking place at these two points. The cells, again, are very small at the head fold, but are very much larger in front of this--larger, in fact, than any other cells of the hypoblast. Outside the embryo they gradually increase in size towards the edge of the pellucid area. Here they are about 1/1000th of an inch in diameter, irregular in shape and rather angular. The outlines of the cells of the epiblast at this time are easily distinguished from the cells of the hypoblast by being more elongated and angular; they are further distinguished by the presence of numerous small oval cells, frequently at the meeting point of several cells, at other times at points along the lines of junction of two cells (Pl. 1, fig. 12). These small cells look very like the smaller stomata of endothelial membranes, but are shewn to be cells by possessing a nucleus. There is considerable variation in size in the cells in different parts of the epiblast. Between the front lobes of the brain the cells are very small, 1/4000th inch, rising to 1/2000th on each side. They are about the latter size over the greater part of the embryo. But over the sinus rhomboidalis they fall again to from 1/3000th to 1/4000th inch. This is probably to be explained by the growth of the medullary fold at this point, which pushes back the primitive groove. At the sides of the head the cells are larger than anywhere else in the epiblast, being here about 1/1000th inch in diameter. I at present see no explanation of this fact. At the periphery of the pellucid area and over the vascular area the cells are 1/1500th to 1/2000th inch in diameter, but at the periphery of the opaque area they are smaller again, being about the 1/3000th of an inch. This smaller size at the periphery of the area opaca is remarkable, since in the earlier stages the most peripheral epiblast cells were the largest. It, perhaps, implies that more rapid growth is at this time taking place in that part of the epiblast which is spreading over the yolk sac. EXPLANATION OF PLATE 1, Figs. 1-5 and 9-12. Fig. 1. Section through an unincubated blastoderm, shewing the upper layer, composed of a single row of columnar cells, and the lower layer, composed of several rows of rounded cells in which no nucleus is visible. Some of the "formative cells," at the bottom of the segmentation cavity, are seen at (_b_). Fig. 2. Section through the periphery of an eight hours' blastoderm, shewing the epiblast (_p_), the hypoblast (_h_), and the mesoblast commencing to be formed (_c_), partly by lower-layer cells enclosed between the epiblast and hypoblast, and partly by formative cells. Formative cells at the bottom of the segmentation cavity are seen at _b_. At _s_ is one of the side folds parallel to the primitive groove.