30 



GENERAL PRINCIPLES OF ZOOLOGY 



FIG. 8. Philicthys xiphice. 

 a, female (after Claus), X4; b, 

 male (after Bergsoe), Xi3. 



The Nervous System. This law applies as well to single organs as to 

 entire animals. The central nervous system of many lower animals 

 (echinoderms, coelenterates, many worms) forms part of the skin; in its 



first appearance it belongs to the surface 

 of the body, because it has to mediate the 

 relations with the external world. In the 

 case of higher animals, e.g., the vertebrates, 

 the brain and spinal cord lie deeply imbed- 

 ded in the interior of the body; but in the 

 embryo they are likewise laid down as a 

 part of the skin (medullary plate) which 

 gradually through infolding and cutting off 

 from this comes to lie internally (fig. 9) . 



The Skeletal System. The skeleton of 

 vertebrates is a further example. In the 

 lowest chordates, amphioxus and the cyclo- 

 stomes, the vertebra are lacking, and in 

 their place we find a cylindrical cord of 

 tissue, the notochord. In the fishes and 

 Amphibia the notochord usually persists; 

 but it is partially reduced and constricted 



by the vertebrae, which in the lower forms consist of cartilage, and in 

 the higher of bone. Mature birds and mammals finally have a com- 

 pletely ossified vertebral column; their embryos, on the other hand, have 

 in the early stages only the notochord (amphioxus stage) ; later this 

 notochord becomes constricted by the vertebrae (fish-amphibian stage) 

 and finally entirely replaced; the vertebral column is in the beginning 

 cartilaginous, only later becoming ossified. Comparative anatomy and 

 embryology thus give the same developmental stages of the axial skeleton: 

 (i) notochord, (2) notochord and vertebral column, (3) vertebral column 

 alone, the latter at first formed of cartilage, then of bone. 



We have spoken of a parallelism between the facts of comparative 

 anatomy and of embryology. But we should expect a threefold parallel- 

 ism; for according to the theory of evolution the systematic arrangement 

 of animals is based upon a third factor phylogeny. The fossils, should 

 give the same progressive series in the successive geological strata as the 

 stages of forms found by comparative anatomy and embryology. We 

 know instances of such threefold parallelisms. Comparative anatomy 

 teaches that the lowest developed form of a fish's tail is the diphyceral 

 (fig. 10, A); that from this the heterocercal (B), and from the hetero- 

 cercal the homocercal form of tail-fin (C, D] can be derived. Embryo- 



