When we open a human embryo of one month (Fig. 183), we find the alimentary canal formed in the body-cavity, and for the most part cut off from the embryonic vesicle. There are both mouth and anus apertures. But the mouth-cavity is not yet separated from the nasal cavity, and the face not yet shaped. The heart shows all its four sections; it is very large, and almost fills the whole of the pectoral cavity (Fig. 183 _ov_). Behind it are the very small rudimentary lungs. The primitive kidneys (_m_) are very large; they fill the greater part of the abdominal cavity, and extend from the liver (_f_) to the pelvic gut. Thus at the end of the first month all the chief organs are already outlined. But there are at this stage no features by which the human embryo materially differs from that of the dog, the hare, the ox, or the horse—in a word, of any other higher mammal. All these embryos have the same, or at least a very similar, form; they can at the most be distinguished from the human embryo by the total size of the body or some other insignificant difference in size. Thus, for instance, in man the head is larger in proportion to the trunk than in the ox. The tail is rather longer in the dog than in man. These are all negligible differences. On the other hand, the whole internal organisation and the form and arrangement of the various organs are essentially the same in the human embryo of four weeks as in the embryos of the other mammals at corresponding stages. Fig.192. Human embryo of the fourth week, one-third of an inch long, lying in the dissected chorion. Fig. 192—Human embryo of the fourth week, one-third of an inch long, lying in the dissected chorion. Fig.193. Human embryo of the fourth week, with its membranes, like Fig. 192, but a little older. Fig. 193—Human embryo of the fourth week, with its membranes, like Fig. 192, but a little older. The yelk-sac is rather smaller, the amnion and chorion larger. It is otherwise in the second month of human development. Fig. 179 represents a human embryo of six weeks (VI), one of seven weeks (VII), and one of eight weeks (VIII), at natural size. The differences which mark off the human embryo from that of the dog and the lower mammals now begin to be more pronounced. We can see important differences at the sixth, and still more at the eighth week, especially in the formation of the head. The size of the various sections of the brain is greater in man, and the tail is shorter. Other differences between man and the lower mammals are found in the relative size of the internal organs. But even at this stage the human embryo differs very little from that of the nearest related mammals—the apes, especially the anthropomorphic apes.