I believe that we can extend this same interpretation to all other cases of embryonic resemblance. It will explain the occurrence of gill-slits in the embryo of the bird, and the presence of a notochord in the higher forms in exactly the same way as the cleavage stages are explained. But how, it may be asked, can we explain the apparent resemblance between the embryo of the higher form and the adult of lower groups. The answer is that this resemblance is deceptive, and in so far as there is a resemblance it depends on the resemblance of the adult of the lower form to its own embryonic stages with which we can really make a comparison. The gill-slits of the embryo of the chick are to be compared, not with those of the adult fish, but with those of the embryo of the fish. It is a significant fact, in this connection, that the gill-slits appear as early in the embryo of the fish as they do in the bird! The notochord of the embryo bird is comparable with that of the embryo of amphioxus, and not with the persistent notochord in the adult amphioxus. Here also it is of the first importance to find that the notochord appears both in the embryo bird and in amphioxus at the very beginning of the development. The embryo bird is not fishlike except in so far as there are certain organs in the embryo fish that are retained in the adult form. The embryo bird bears the same relation to the embryo fish that the early segmentation stages of the mollusk bear to the early segmentation stages of the annelid. There are certain obvious resemblances between this view and that of Von Baer, but there are also some fundamental differences between the two conceptions.
Von Baer thought that within each group the embryonic development is the same up to a certain point. He supposed that the characters of the group are the first to appear, then those of the order, class, family, genus, and, finally, of the species. He supposed that two similar species would follow the same method of development until the very last stage was reached, when each would then add the final touches that give the individual its specific character. We may call this the theory of embryonic parallelism. Here there is an important difference between my view and that of Von Baer, for I should not expect to find the two embryos of any two species identical at any stage of their development, but at most there might exist a close resemblance between them.
