98 PROP. MARSHALL AND MR. SPENCER. 



this stage and the earlier one which we have more fully described. 

 We have traced all these nerves up to what is practically the adult 

 condition, and have identified them with the branches bearing the 

 same names in the adult. Our observations show that in the fifth, as 

 in the third nerve, all the main branches of the adult nerve are fully 

 established by stage l, and that the after changes are comparatively 

 unimportant. 



The Seventh, or Facial Nerve. 



a. The roots of origin. — Fig. 2 represents a transverse section 

 through the roots of origin of the seventh nerve of an embryo between 

 stages i and k, the same, in fact, of which fig. 1 represents the roots 

 of the fifth nerve. The nerves (vn) are seen to arise from the 

 extreme dorsal summit of the hind-brain, the roots of origin of the 

 two (vii a) being continuous with one another across the top of the 

 brain. It will also be noticed, as contested with figure 1, that 

 the two sides of the hind-brain are close together, both above and 

 below, and that the cavity of the hind-brain is a mere vertical slit ; 

 that, in fact, the growth of the roof of the brain, which we have seen 

 is the first cause of the separation of the roots of the fifth, has not yet 

 commenced in the part of the brain from which the seventh nerves 

 arise. The section further shows that the nerve on either side extends 

 down as a somewhat club-shaped mass of compactly arranged polygonal 

 cells lying between the external epiblast and the neural canal, but 

 distinct from both, its ventral end having a tendency to pass to the 

 outer side of the third head cavity (fig. 2, 3), just as the fifth nerve 

 passed to the outer side of the second cavity (fig. 1, 2). 



The next stage is represented in fig. 3, a section through the hind 

 brain and roots of the seventh nerve of an embryo of stage K, of the 

 same age though not from the same specimen as fig. 4. The figure 

 shows that very important changes have occurred; the roof of the 

 hind-brain has grown rapidly and considerably, so as to separate 

 widely the two primary roots of the seventh nerves (vii a). On the 

 right side this dorsal primary root is alone seen, but on the left side a 

 considerable portion of the nerve is shown, and it is seen that, in 

 addition to the primary root (vii a), which is still present, the nerve has 

 acquired a new or secondary root (vii /3) about half way down the side 

 of the brain. Both roots of attachment are perfectly clear and unmis- 

 takeable, while between them the nerve and brain are quite distinct 

 from one another, and separated by an appreciable interval, 



