VESTIGIAL PARTS. 75 



sistence of the gut as a simple tube, with an accessory 

 opening near the coccyx, to large growths exceeding 

 the weight of the child unfortunate enough to possess 

 so unwelcome an appendage. 



The transformation of a piece of intestine into a 

 central nervous system has had the effect of rendering 

 vestigial, structures which not unfrequently behave in 

 a manner pernicious to the individual. 



For instance, the development of a spinal column to 

 protect the cord is an outcome of this transformation, 

 and the various defects in the development of the cord 

 and column, if serious, are incompatible with life. These 

 defects, known collectively as spina bifida, are of such 

 frequent occurrence that in a recent careful scientific 

 report upon this subject, it appears, that in England 

 alone, six hundred and forty-seven deaths occurred in 

 1882 from this malformation, of which six hundred and 

 fifteen were in children under one year of age. It would, 

 in a work of this kind, be difficult to enter fully into 

 details of the various forms of this interesting class of 

 defects, but it may be briefly stated that many of them 

 are failures in the formation of the bony walls, others 

 are due to protrusions of the spinal membranes, and a 

 rarer form arises in consequence of an accumulation of 

 fluid inducing local dilatation of the central canal of the 

 cord : an example of the cystic dilatation of a function- 

 less canal. 



'That remarkable appendage of the developing ali- 

 mentary canal, the yolk-sac, with its vitello-intestinal 

 duct, has already been referred to ; its significance is 

 not easy to estimate. Although the duct connecting it 



