194 ARTHUR E. SHIPLEY. 



described by Calberla, Scott, and others, and I have nothing 

 to add to their account. In the histological differentiation of 

 the chord from a solid string of more or less cubical cells, to 

 the vacuolated cylinder which forms the permanent notochord, 

 there is a stage which is perhaps worth mentioning. In the early 

 stages a transverse section of the chord shows portions of three 

 or four cells, a little later these cells have pushed their way 

 between one another and arranged themselves in such a way 

 that they occupy the whole room inside the sheath of the 

 notochord. Whilst in this condition vacuoles appear in the 

 substance of the cells and for a day or two the notochord pre- 

 sents very much the same structure as the notochord of Amphi- 

 oxus. This is, however, soon replaced by the vacuolated 

 appearance characteristic of the notochordal tissue of the higher 

 Vertebrata (figs. 18 and 23). 



The posterior end of the notochord passes into the indifferent 

 mass of tissue described in the tail. The anterior end is 

 slightly curved downwards apparently by the increased vertical 

 height of the brain. It ends just behind the infundibulum, 

 its end being in contact with the posterior end of the nasal in- 

 vagination. There is no trace that it has ever passed in front 

 of this point, although in the young stages it reaches relatively 

 almost as far forward as the nervous system. The relation of 

 its anterior end to the brain hence appears to be due to the 

 overgrowth of the nervous system anteriorly. 



The cartilage which composes the rest of the skeleton is 

 characterised by the small amount of intercellular substance. 

 This stains very deeply. The cells are large with usually only 

 one nucleus, though sometimes two. I have endeavoured to 

 represent this structure in fig. 19. The branchial bases are 

 the first part of the skeleton to appear. They arise about the 

 twenty-fourth day as straight bars of cartilage lying external 

 and slightly posterior to the branchial vessel. In their relation 

 to the vessel they correspond with the extrabranchial bars of 

 the Tadpole, and the Sharks. The true branchial bars run 

 internal to the branchial vessel. 



The bars run behind the gill-slit to which they belong, and 



