Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/182

132 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 composed of a number of bony segments or vertebrae. 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 condition of the bodies of the vertebrae, is retained permanently, however, in the Amphioxus and Myxinoid fishes. The Chorda Dorsalis, until recently, was supposed to characterize the Vcrtebrata, 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 in the Amphioxus, the simplest of fishes. The importance of this discovery cannot be exaggerated, as the embryo Ascidian furnishes the transition from the Invertebrata 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.