136 Elementary Zoology. 



since no use can be assigned to them, and so account for their 

 presence. 



The notochord is an organ of a similar character. In the 

 Amphioxus the notochord is the only part of the skeleton to 

 appear at all, and it persists throughout life. In the chick the 

 notochord is replaced by (is not developed into) the backbone. 

 This is an interesting case of a frequent phenomenon of what 

 has been termed the " Substitution of organs." Very often it 

 happens that an organ is, as it were, manufactured out of 

 another organ. An example of this is the formation of the 

 sternum out of the ends of the fused ribs, or of the auditory 

 passage out of the first gill-cleft. But sometimes a new organ, 

 having a similar function to one already existing, is formed 

 afresh in the same place. Thus the backbone is not a product 

 of the notochord, but it appears at the same spot, and performs 

 much the same function, strengthening the body in a longi- 

 tudinal direction. In the one case — to use a homely simile — 

 an old coat is cut down into a waistcoat; in the other, an 

 entirely new coat of a different pattern is acquired. 



Development of the Organs of the Body. 



It will be more convenient to deal with the organs 

 separately, instead of following a strictly chronological order 

 and pursuing the course of development of all the systems 

 together. 



From the epiblast are formed the skin, i.e. the epidermis; 

 the central nervous systein (as already mentioned) ; the stomo- 

 dcBum and proctodceum ; portions of the eye apd of the ear ; and 

 the olfactory organs. 



The stomodaum and the proctodceum are involutions of the 

 epiblast to forni the lining membrane of the first and last 

 parts of the alimentary tract. The greater part of the lining 

 epithelium of this tract is hypoblastic. But the mouth cavity 

 is entirely epiblastic, while the posterior aperture is formed 

 by a very short invagination of epiblastic cells. 



It seems clear that all sense organs should be primitively 



