Vertebra and Annellus. 61 



of the protovertebra coalesce the chorda dorsalis disap- 

 pears: and, in short, the whole history of the chorda 

 dorsalis is that of a part in which development does not 

 take place, rather than that of a definite centre of 

 development that of a mere pith-cavity, it may be. 

 Nor is this conclusion invalidated by anything that is 

 met with in creatures where, as in the lamprey, the 

 chorda dorsalis remains permanently : for the place of 

 the chorda dorsalis in the dried skeleton in these 

 creatures is occupied by a hollow space with some 

 scant remains of shrivelled soft tissue in it, the process 

 of drying having brought about a change which would 

 seem to be not unlike that which happens to Meckel's 

 cartilage on drying an embryonic lower jaw bone. 

 Everything, indeed, goes to show that the " body " of the 

 vertebra, instead of occupying a central position primarily, 

 is formed by the secondary coalescence of certain parts 

 belonging to each of the lateral halves of the protover- 

 tebra. So it is certainly in the chick and pig, and so it 

 is also, so far as is known, in vertebrate animals gene- 

 rally. Nor is the case otherwise with the cephalic portion 

 of the vertebral column. Here there is no very distinct 

 chorda dorsalis for this part does not extend beyond 

 the spinal portion of the vertebral column and no very 

 distinct division into protovertebrae, but there are even- 

 tually two lateral ridges, called traberculae, which diverge 

 and meet again so as to enclose a space called the 

 pituitary space. Here, indeed, there is strong confirma- 

 tory evidence in favour of the conclusion already drawn 

 respecting the nature of the chorda dorsalis, and the 

 mode of formation of the " bodies " of the vertebrae, for 

 little imagination is required to see how the pituitary 

 space may agree with the space of the chorda dorsalis, 

 and the soft pituitary body with the soft contents of the 



