342 PHOGRESS OF SCIENCE IN THE CENTURY. 



chord, — a rod developed along the dorsal wall of the 

 gut. From Fishes onwards, this embryonic axis is 

 gradually replaced in development by the vertebral 

 column or backbone; the notochord does not become 

 the backbone, but is replaced by it. It is a tem- 

 porary structure, around which the vertebral column 

 is constructed, as a tall chimney may be built around 

 an internal scaffolding of wood. Yet, it remains 

 as the sole axial skeleton in Amphioxus, likewise in 

 great part in hag and lamprey, but becomes less and 

 less persistent in Fishes and higher vertebrates, as 

 its substitute, the backbone, develops more perfectly. 

 Now, what is the relation between the notochord and 

 its substitute, the backbone, seeing that the former 

 does not become the latter '? Kleinenberg's suggestion 

 is that the notochord supplies the stimulus, the neces- 

 sary condition, for the formation of the backbone. 

 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 suggestive. 

 It is consistent with our general conception of de- 

 velopment — that each stage supplies the necessary 

 stimulus for the next step ; it also helps us to under- 

 stand more clearly how new structures, too incipient 

 to be of use, may persist. 



Rudimentary Organs. — In many animals there 

 are structures which attain no complete development, 

 which are rudimentary in comparison with those of 

 related forms, and seem retrogressive when compared 

 with their promise in embryonic life. But it is neces- 

 sary to distinguish various kinds of rudimentary 

 structures, (a) As a pathological variation, probably 

 due to some germinal defect, or to the insufficient 

 nutrition of the embryo, the heart of a mammal is 



