98 AN INTRODUCTION TO SCIENCE 



exterior ; and the phenomenon which all zoologists have had 

 to try to explain is the formation of a new mouth which 

 occurs at the anterior end of the vertebrate embryo, perfo- 

 rating through into the so-called fore- gut. 



Taking the widest view of these and of many other 

 differences in structures which distinguish the vertebrate 

 from the invertebrate, Dr. Gaskell has offered us a startling 

 explanation of the transformation which has occurred. All 

 those parts of the invertebrate body which are median and 

 impaired all that makes up the body of the sea-anemone, 

 that is to say, but not the limbs of insects, lobsters, and 

 other bilaterally symmetrical animals lie on the dorsal 

 side of the vertebral column. Its invertebrate alimentary 

 canal is our neural canal. Its stomach is the ventricular 

 cavity in our brain its gullet passed through a hole (the 

 pituitaiy fossa) in the base of our skull. Every anatomist 

 recognises that the central nervous system is the most con- 

 servative system in the body. It is the first part to be 

 formed in the embryo, the last to follow the changes in 

 other organs of the body. Nerves may change their course, 

 but their centres in the cerebro-spinal axis remain unaltered. 

 The whole animal may alter in appearance, but the nervous 

 system is not essentially affected. It is the central system 

 about which the rest of the body grows ; and there can be 

 little doubt that the central nervous organs in man are 

 homologous with those of arthropods or molluscs, little as 

 any other part of our body finds its counterpart in these 

 animals. In the invertebrate the central nervous system 

 consists of a collar round the oesophagus, certain ganglia in 

 the head, and a double chain of ganglia along the ventral 

 side of the alimentary canal. These ganglia, says Dr. 

 Gaskell, which have already coalesced in the highest inver- 

 tebrates, become the brain and spinal cord. Growing vastly 

 in importance as the animal series is ascended, they have 

 grown round and blocked in its primitive alimentary canal. 



If we wish to trace the history of the greater part of our 



