206 AliTHUR E. SHIPLEY, 



lateral ventricles are to be seen. The lateral outgrowths of 

 the hemispheres embrace between them a mass of tissue formed 

 at the back of the olfactory pit, which resembles in every way 

 nerve matter. This structure is shown in figs. 33, 34, and 35, 

 drawn from a series of sections taken through the head of a 

 fifty-two days' larva. This tissue in question appears to con- 

 sist of ganglion cells. It is traversed by a canal which ends 

 blindly behind and opens by the median nasal pit in front. 

 Posteriorly it is continuous with a sheet of tissue which is de- 

 scribed by Dohrn and Scott as giving rise to the pituitary 

 body (fig. 39). Unfortunately my larvae were not sufficiently 

 old to enable me to determine whether this mass of tissue 

 comes into closer relation with the brain and forms the olfac- 

 tory lobes, or whether, as seems more probable from what we 

 know of the development of these structures in other animals, it 

 forms only the peripheral portion of the olfactory apparatus. 



About the twenty-fifth day some of the ganglion cells in the 

 postero lateral angle of the grey matter become much larger 

 than the surrounding ones. These cells are particularly fre- 

 quent in that part of the hind-brain lying between the audi- 

 tory capsule. They probably develope into the " outer large 

 cells" of Reissue r. 



With regard to the development of the cranial nerves, I 

 have no observations on the origin of the olfactory nerve, as 

 this apparently does not arise till a much later stage than that 

 attained by my oldest larvae. The origin of the optic nerve as 

 an outgrowth of the brain has been described above. Owing 

 to the rudimentary condition of the eye, the muscles of that 

 organ are not developed, and consequently the third, fourth, 

 and sixth nerves do not arise till a much later stage. This 

 leaves the fifth, seventh, eighth, ninth, and tenth nerves to be 

 considered. 



Tlie origin of these nerves is much obscured by the yolk 

 which crowds the cells of the embryo at the time they first 

 appear. On the seventeenth day the first origin of the ganglia 

 in the fifth and seventh nerve is seen. The ganglia arise as 

 proliferations of the epiblast. By this means a knob of cells 



