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 spzna 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 
