370 Zoological Classification. 



formed between the parallel ridges thus developed, and the mar- 

 gins of these, eventually uniting with one another, constitute a 

 second tube parallel with the first, by a modification of the 

 inner walls of which, the vertebrate cerebro-spinal nervous 

 centre is developed. Hence it follows, that after any verte- 

 brate animal has passed through the very earliest stages of its 

 development, it is not a single, but a double tube, and the 

 two tubes are separated by a partition which was primitively 

 a part of the external parietes of the body, but which now lies 

 in a central position between the cerebro-spinal nervous cen- 

 tres and the alimentary canal." Hence a diagrammatic and 

 transverse section of a vertebrate animal exhibits two tubes, 

 one containing the alimentary canal, the heart and certain 

 nervous centres belonging to the so-called sympathetic system, 

 and the other containing the cerebro-spinal tube, " which 

 appears to be a superaddition — a something not represented 

 in the invertebrate series." Several other highly important 

 differences separate the vertebrates from the invertebrates; 

 but shall now only mention that a vertebrate animal never has 

 more than two pairs of limbs, that it has a distinct vascular 

 system containing blood, " with suspended corpuscles of one 

 kind, two kinds, or even three distinct kinds/-' and its jaws 

 are modifications of the cephalic parietes, and do not resemble 

 the masticatory apparatus of the invertebrate, which are " either 

 hard productions of the alimentary mucous membrane, or mo- 

 dified limbs." 



It is remarkable that not only do the early condition of 

 the vertebrates mark them out from the invertebrates, as just 

 explained, but peculiarities of this condition also distinguish 

 the vertebrates from each other. Before we give any illustra- 

 tion of this we will copy from Professor Huxley a lucid 

 description of the amnion and the allantoic. " The amnion," 

 he says, " is a sac filled with a fluid which envelopes and 

 shelters the embryo during its slow assumption of the condi- 

 tion in which it is competent to breathe and receive food from 

 without. . . . The allantois is developed much later than 

 the amnion, neither from the serous, nor from the mucous (or 

 epidermic and epithelial) layers of the germs ; but from that 

 intermediate stratum whence the bones, muscles and vessels 

 are evolved. It arises, as a solid mass, from the under part of 

 the body of the embryo, behind the primitive intestinal cavity ; 

 and, enlarging, becomes a vesicle, which rapidly increases in 

 size, envelopes the whole embryo, and being abundantly sup- 

 plied with arterial vessels from the aorta, serves as the great 

 instrument of respiration during foetal life." In the bird, 

 " the porosity of the egg-shell allows the allantoic blood to 

 exchange its excess of carbonic acid for oxygen by osmosis." 



