The point to which I wish to draw especial attention in this connection is that in the higher forms the gill-slits appear at a very early stage; in fact, as early in the mammal as in the salamander or the fish, so that if we suppose their appearance in the mammal is a repetition of the adult amphibian stage, then, since this stage appears as early in the development of the mammal as in the amphibians themselves, the conclusion is somewhat paradoxical.
The history of the notochord in the vertebrate series gives an interesting parallel. In amphioxus it is a tough and firm cord that extends from end to end of the body. On each side of it lie the plates of muscles. It appears at a very early stage of development as a fold of the upper wall of the digestive tract. In the cartilaginous fishes the notochord also appears at a very early stage, and also from the dorsal wall of the digestive tract. In later embryonic stages it becomes surrounded by a cartilaginous sheath, or tube, which then segments into blocks, the vertebræ. The notochord becomes partially obliterated as the centra of the vertebræ are formed, but traces of it are present even in adult stages. In the lower amphibians the notochord arises also at an early stage over and perhaps, in part, from the dorsal wall of the digestive tract. It is later almost entirely obliterated by the development of the vertebræ. These vertebræ first appear as a membraneous tube which breaks up into cartilaginous blocks, and these are the structures around and in which the bone develops to form the permanent vertebræ.
In higher forms, reptiles, birds, and mammals, the notochord also appears at the very beginning of the development, but it is not certain that we can call the material out of which it forms the dorsal wall of the archenteron (the amphibians giving, perhaps, intermediate stages). It becomes surrounded by continuous tissue which breaks up into blocks, and these become the bases of the vertebræ. The notochord becomes so nearly obliterated in later stages that only the barest traces of it are left either in the spaces between, or in, the vertebræ.
In this series we see the higher forms passing through stages similar at first to those through which the lower forms pass; and it is especially worthy of note that the embryo mammal begins to produce its notochord at the very beginning of its development, at a stage, in fact, so far as comparison is possible, as early as that at which the notochord of amphioxus develops.