ON EMBRYOLOGY AS AN AID TO ANATOMY 51 



much larger and more conspicuous in the brain than 

 elsewhere, but occur along the whole length of the 

 cord. The first dilatation is the fore brain, and 

 from it the cerebral hemispheres project ; the 

 second is the mid brain ; this is followed by a series 

 of vesicles, rapidly decreasing in size, and called 

 collectively the hind brain ; behind this we have the 

 spinal cord. In the spinal cord the dilatations, 

 though only feebly marked, manifestly correspond 

 to the segments, and from each one a pair of spinal 

 nerves arises. Similarly in the brain from each 

 well-marked vesicle a pair of nerves arises. Thus, 

 from the mid brain the third pair of nerves arises 

 and runs backwards to the interval between the 

 first and second head cavities. From the first 

 vesicle of the hind brain arises the fifth nerve, and 

 this passes down between the second and third 

 head cavities. From the second vesicle of the hind 

 brain the seventh or facial nerve springs and runs 

 down behind the third head cavity. The division 

 of the head into segments is thus very clearly and 

 satisfactorily shown, for all the different elements 

 that could afford evidence viz., brain vesicles, 

 nerves, head cavities, and visceral clefts and arches 

 all point to the same divisions and agree among 

 themselves. 



We have now got all the links of our evidence 

 nearly complete ; let us fit them together and give 

 them the finishing strokes. We have seen that the 

 walls of the head cavities like their homologues in 

 the body develop into muscles. Now the first head 

 cavity lies immediately behind the eye and extends 



