196 



GENERAL BIOLOGY 



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simultaneously. The first to appear externally is the 

 nervous axis upon the dorsal side and this and the gill slits 



and the notochord are es- 

 pecially worthy of our consid- 

 eration. 



Beside the blastopore an 

 elevation appears which rises 

 in two parallel ridges called 

 the neural folds. These folds 

 at their outgrowing ends are 

 confluent in a loop which 

 marks the head end of the 

 salamander. These folds grow 

 rapidly, and by their increase 

 in length project from the sur- 

 face of the egg at both ends, 

 spoiling its spherical symme- 

 try. The withdrawal of the 

 yolk plug is accompanied by 

 the outpushing of the tail be- 

 yond the end of the neural 

 folds. The folds come togeth- 

 er, as indicated in the accom- 

 panying figures, the wider an- 

 terior end becoming the brain, 

 the remainder, the spinal cord. 

 This then sinks into the in- 

 terior as indicated in the 

 cross-section diagrams of fig- 

 ure 122. 



Meanwhile the archenteron is elongating in a parallel 

 direction, and a fold from its dorsal wall is cutting off the 

 notochord — the most ancient supporting structure of verte- 

 brates — a larval organ in most of them at the present day. 



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