In embryos of from thirty to forty hours the cells of the hypoblast have, over the central portion of the pellucid area, entirely lost their highly refractive spherules, and in the fresh state are composed of the most transparent protoplasm. When treated with reagents they are found to contain an oval nucleus with one or sometimes two nucleoli, imbedded in a considerable mass of protoplasm. The protoplasm appears slightly granular and generally contains one or two small vacuoles. I have already spoken of the gradation of the hypoblast at the edge of the blastoderm into white yolk. I have, therefore, only to mention the variations in the size of its cells in different parts of the pellucid area. The points where the cells are smallest seem generally to coincide with the points of maximum growth. Over the embryo the cells are more regular than elsewhere. They are elongated and arranged transversely to the long axis of the embryo. They are somewhat hexagonal in shape, and not unlike the longer pieces in the dental plate of a Myliobatis (Pl. 1, fig. 10). This regularity, however, is much more marked in some specimens than in others. These cells are about 1/4000th of an inch in breadth, and 1/1000th in length. On each side of the embryo immediately external to the protovertebræ the cells are frequently about the same size as those over the embryo itself. In the neck, however, and near the end of the sinus rhomboidalis, they are considerably smaller, about 1/4000th inch each way. The reason of this small size is not very 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.