684 LIFE : OUTLINES OF GENERAL BIOLOGY 



replaced by the vertebral column or backbone, which develops 

 from a mesodermic sheath formed around the notochord. The noto- 

 chord does not become the backbone ; it is a temporary endodermic 

 structure, around which the permanent vertebral column is con- 

 structed, as a tall brick chimney might be built around an internal 

 scaffolding of wood. But what is the relation between the noto- 

 chord and its substitute the backbone? Kleinenberg's suggestion is 

 that the notochord in some way supplies the stimulus, the neces- 

 sary precondition, for the development of the backbone. There is 

 no doubt that one part affects another in embryonic development, 

 sometimes by way of inhibition, as the growing point of a stem 

 influences the buds just below; sometimes, by way of stimulus, as 

 when the ingrowth of the ectodermic dental germ stimulates the 

 development of a mesodermic papilla that forms the bulk of the 

 tooth. The cerebral diverticulum that forms the foundation of the 

 eye in Vertebrates grows till it meets the ectoderm, which then 

 proliferates to form the lens; and whatever the nature of the 

 stimulus may be, it is so powerful and specific that the experimental 

 introduction of a fragment of optic vesicle underneath the epidermis 

 of the very young tadpole will induce the formation of a lens in an 

 entirely irrelevant situation, even beyond the limits of the head. 

 It may be that an influence of this sort — possibly analogous to 

 that of a hormone — is exerted by the endodermic notochord on the 

 adjoining mesodermic tissue so that the development of a back- 

 bone is induced. The idea helps towards an understanding of the 

 persistence of incipient evolving structures during the period before 

 they are large enough to be of use. It also throws light on the pro- 

 longed persistence of dwindling structures which are relatively large 

 in the developmental stages, but eventually of no fimctional 

 importance. 



CHANGE OF FUNCTION IN ORGANS.— One method of evo- 

 lution is to effect a gradual change in the function of an organ. 

 Different stages in the transformation can often be traced in a 

 series of related types. Thus the Eustachian tube which leads from 

 the outer ear-passage to the back of the mouth in all animals from 

 Amphibians upwards is homologous with the first gill-cleft or 

 spiracle of a cartilaginous fish, e.g. the skate, by which water enters 

 the pharynx to pass out by the other gill-clefts, washing the gills on 

 the way. There has been a notable change of function, for the 

 Eustachian tube is not connected with respiration. In many cases, 

 however, an organ has several functions, and the change that 

 comes about is that a secondary function becomes in the course of 

 time the dominant function. This idea of "Functionswechsel" 

 received much attention from Dr. Anton Dohm, the distinguished 

 founder of the Naples Zoological Station, who spoke of it thus: 



