132 EVOLUTION OF 0 
senting the embryo Turtle, Chicken, Dog, and Man, illustrate 
the resemblance of vertebrate animals at an early stage of 
their existence. Not only, however, does man at such a 
period resemble a Turtle, and is undistinguishable from a 
Dog, but the transitory stages of his internal organization 
are also more or less represented as permanent structures 
in the lower animals. This generalization, which is one of 
the most important in Biology, may be expressed in the 
statement, that the structures which are transitory in the 
higher animals are retained permanently in the lower. 
Thus, for example, the spine of the higher animals is com- 
posed of a number of bony segments or vertebra. These 
are represented in the embryo, however, by a cylindrical 
rod of cells, the Chorda Dorsalis, and by a few quadrate 
masses lying on each side of the central nervous system. 
The Chorda Dorsalis, which is only the rudimentary con- 
dition of the bodies of the vertebree, is retained permanently, 
however, in the Amphioxus and Myxinoid fishes. The 
Chorda Dorsalis, until recently, was supposed to charac- 
terize the Vertebrata, and as it is a very important structure, 
its apparent absence in the Invertebrata (animals without a 
backbone) was often urged as an insuperable objection to 
the view of the higher forms of life having come from the 
lower. The free-swimming embryos of the Ascidian worms, 
however, according to Kowalebsky and others, exhibit, in 
their organization, a Chorda Dorsalis (Fig. 38 a, C) and a 
Central nervous system, which develop in the same manner 
as that observed inthe Amphioxus, the simplest of fishes. The 
importance of this discovery cannot be exaggerated, as the 
embryo Ascidian furnishes the transition from the Inverte- 
brata to the Vertebrata. We have seen that the Central 
nervous system is formed through the conversion of the 
Primitive groove into a tube. The tube is originally pointed 
at both ends, and this rudimentary condition is retained 
permanently in the spinal marrow of the Amphioxus (Fig. 
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