30 THE ELEMENTS OF STRUCTURE. 



by Kleinenberg. An illustration will explain it better than a 

 concise statement. In the simplest Chordata, young Tuni- 

 cates and Amphioxu%, and in the early stages of all vertebrate 

 embryos, the supporting skeleton is a dorsal rod or notochord, 

 developed along the dorsal wall of the gut. In Vertebrates, 

 from Fishes onwards, this embryonic axis is replaced by the 

 vertebral column or backbone ; it does not become, but 

 is replaced by the backbone. It is a temporary structure, 

 around which the vertebral column is constructed, as a tall 

 chimney may be built around an internal scaffolding of wood. 

 Yet, as we have said, it remains as the axial skeleton in 

 Amphioxus, likewise in great part in hagfish and lamprey, 

 but becomes less and less persistent in adult life, as its 

 substitute, the backbone, develops more perfectly in Fishes 

 and higher Vertebrates. Now, what is the relation between 

 the notochord and its substitute the backbone, seeing that 

 the fgrmer certainly does not become the latter ? Kleinen- 

 berg's suggestion is that the notochord supplies the stimulus, 

 the necessary condition, for the formation of the cartilaginous 

 and eventually bony vertebral column. Of course, we 

 require to know more about the way in which an old-fashioned 

 structure may stimulate the growth of its future substitute, 

 but the general idea of one organ leading on to another is 

 clear and suggestive. It is consistent with the vague idea 

 which we all have, that every stage in development supplies 

 the necessary stimulus for the next step ; moreover, it 

 enables us to understand more clearly the persistence of 

 new structures too incipient to be of use. 



Rudimentary Organs. — In many animals there are struct- 

 ures which attain no complete development, which are 

 rudimentary in comparison with those of related forms, and 

 are retrogressive when compared with their promise in 

 embryonic life. It is necessary to distinguish various kinds 

 of " rudimentary structure." (a) As a pathological variation, 

 probably due to some germinal defect, or to the inefificient 

 nutrition of the embryo, the heart of an individual mammal is 

 sometimes incompletely formed. Other organs may be simi- 

 larly spoilt in the making. We may call these illustrations of 

 arrested development, {b) Some animals lose, in the course 

 of their life, some of the most promiseful characteristics of 

 their la'rval life : thus parasitic Crustaceans at first free-living 



