xi THE ELEMENTS OF STRUCTURE 211 



a consummate nose. The spinnerets of spiders are 

 exquisite contrivances for the issue of jets of liquid silk, 

 but they are morphologically equivalent to two or three 

 pairs of abdominal limbs, and it is instructive to notice 

 that the embryo spider has three or four other pairs of 

 abdominal appendages which do not become limbs in the 

 adult. The sting of a bee is a specialised ovipositor (and 

 therefore unrepresented in the drone) ; the fang of a 

 venomous snake is a tooth folded so as to enclose a canal, 

 and the bag of poison is an evolved salivary or labial 

 gland. The Eustachian tube which extends from the 

 tympanic cavity of the ear to the back of the mouth is 

 but a transformed and persistent gill-cleft (the spiracle 

 of the skate), and the three-linked chain of ossicles which 

 convey vibrations from the drum or tympanum to the 

 internal organ of hearing was once in whole or in part 

 included in the commonplace framework of the jaws. 



9. Substitution of Organs.- -To the embryologist 

 Kleinenberg we owe a suggestive conception of organic 

 change, which he spoke of as the development of organs 

 by substitution : An organ may supply the stimulus and 

 the necessary condition for another which gradually 

 supersedes and replaces it. In the simplest backboned 

 animals, such as the lancelet, there is a supporting- 

 skeletal rod along the back ; among fishes the same 

 rod or notochord is largely replaced by a backbone ; in 

 yet higher Vertebrates the adults have almost no vestige 

 of notochord, its replacement by the backbone is almost 

 complete. So in the individual life-history, all verte- 

 brate embryos have a notochord to begin with ; in the 

 lancelet and some others this is retained throughout life, 

 in higher forms it is temporary and serves as a scaffolding 

 around which, from a thoroughly distinct embryological 

 origin, the backbone develops. What is the relation 

 between these two structures notochord and backbone ? 

 According to Kleinenberg, the notochord supplies the 

 necessary stimulus or condition for the development of 

 the backbone which replaces it. 



The general idea of one organ leading on to another is 

 suggestive. It is consistent with our general conception 



