38 



GENERAL PRINCIPLES OF ZOOLOGY. 



pass througli tho stage of organization of the lower, because 



they spring from ancestors wliicli were 

 more or loss similar to the latter. Man 

 in his emhryological 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 

 wore once fish-like, perennibranchiate- 

 like, nauplius- and cyclops-like. Here 

 is expressed a general phenomenon 

 which Ilaeckel has stated in a general 

 proposition under the name of ' tlie 

 Fundamental Law of Biogenesis. ' ' ' The 

 development history (ontogeny) of an 



V individual animal briefly recapitulates 

 the history of the race (phvlogeny); 



Fig. S.—Plnlicthys xipMce. 

 male (after Claus), X 

 (after Bergsoe), X 13. 



male (after Claus), x 4; ft, male' 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. — The Xerrous 

 I'^i/sfein. — This law applies as well to single organs as to entire 

 animals. The central nervous system of the lower animals 

 (echinoderms, crelonterates, 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; biit 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 of 

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

 different ages of any verteljrate (fig. '.!)• 



The SkcIeldJ Systeii). — The skeleton of vertebrates is a further 

 example. In the lowest chordates, ampliioxus and the cijelostomes, 

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

 cord of tissite, tlie chorda dorsalis (notochord). In the fishes and 

 Aniphil)ia tho notochord usually persists; Init it is partially 

 reduced and constricted by the vortel)r;i5, which in the lower forms 

 consist of cartilage, and in tlie higher of bone or a combination of 

 ])one and cartilage, ilaturo birds and mammals rinallv have a 



