The T: heory of Evolution 65 
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 vertebre. The noto- 
chord becomes partially obliterated as the centra of the 
vertebree 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 vertebra. These 
vertebrz 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 per- 
manent vertebree. 
In higher forms, reptiles, birds, and mammals, the noto- 
chord also appears at the very beginning of the develop- 
ment, but it is not certain that we cancall the material out 
of which it forms the dorsal wall of the archenteron (the 
amphibians giving, perhaps, intermediate stages). It be- 
comes surrounded by continuous tissue which breaks up into 
blocks, and these become the bases of the vertebrae. 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 vertebrz. 
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 begin- 
ning of its development, at a stage, in fact, so far as compari- 
son is possible, as early as that at which the notochord of 
amphioxus develops. 
The development of the skull gives a somewhat similar 
case. The skulls of sharks and skates are entirely cartilagi- 
nous and imperfectly enclose the brain. The ganoids 
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