GENERAL PRINCIPLES OF ZOOLOGY. 



Fl mai 8 eTa P f?ef ffius x 

 (after Bergsoe), x 13. 



pass through the stage of organization of the lower, because 



they spring from ancestors which were 

 more or less similar to the latter. Man 

 in his embryological development passes 

 through the fish stage, the frog the per- 

 ennibranchiate stage, the parasitic crus- 

 tacean first the nauplius- and then the 

 cyclops-stage, because their ancestors 

 were once fish-like, perennibranchiate- 

 like, nauplius- and cyclops-like. Here 

 is expressed a general phenomenon 

 which Haeckel has stated in a general 

 proposition under the name of 'the 

 Fundamental Law of Biogenesis. ' ' ' The 

 development history (ontogeny) of an 

 individual animal briefly recapitulates 

 the history of the race (phylogeny); 

 i. e. , the most important stages of organi- 

 zation which its ancestors have passed 

 through appear again, even if somewhat modified, in the develop- 

 ment of individual animals." 



Examples of the Application of this Law. Hie Nervous 

 System. This law applies as well to single organs as to entire 

 animals. The central nervous system of the 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 embedded in the interior of the body; but in the 

 embryo it is laid down likewise as a part of the skin (medullary 

 plate) and which gradually through infolding and cutting off from 

 this comes to lie internally. One can demonstrate this change oi 

 position by cross-sections through the dorsal region of embryos of 

 different ages of any vertebrate (fig. 9). 



The Skeletal System. The skeleton of vertebrates is a further 

 example. In the lowest chordates, amphioxus and the cyclostomes, 

 the vertebrae are lacking, and in their place we find a cylindrical 

 cord of tissue, the chorda dorsalis (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 or a combination of 

 bone and cartilage. Mature birds and mammals finally have a 



