DESCRIPTION OF PLATES V AND Vl.-^Contimed. 



ophthalmic branches of fifth aud seventh, first head cavity, and rectus 

 externus and obliquus inferior muscles. 

 Figs. 21 to 25. — Horizontal sections through the head of an embryo at a 

 stage intermediate between l and M. x 20. 



Fig. 21. The most ventral of the series ; shows first head cavities, 

 iofundibulum, and pituitary involution. 



Fig, 22 shows first and second head cavities, with prolongation of 

 latter into mandibular arch ; also whole length of third nerves on both 

 sides, and ciliary ganglion. 



Fig. 23 shows tlie median portion of first head cavity, second head 

 cavities, ciliary ganglia, and fiftli nerves. 



Fig. 24 shows third head cavities, fifth nerve with ophthalmic and 

 communicating branches, seventh uerve, and ventral portions of third head 

 cavities in hyoideau arches. 



Fig. 25 shows third head cavities, fifth, seventh, and auditory nerves, 

 with ophthalmic branches of seventh. 

 Figs. 26 to 31. — Transverse sections through the head of an embryo at a 

 stage intermediate between M and N. X 25. 



Fig. 26, the most anterior of the series, shows first head cavities, 

 ciliary ganglia, fifth nerve with its anterior roots, ophthalmic and com- 

 municating branches, and the oplitlialmic branches of the seventh nerve. 



Fig. 27 shows first head cavities, filth nerve with ganglionic root on 

 left side, nou-ganglionic root on right side, ciliary ganglia, and ophthalmic 

 branches of seventh. 



Fig. 28 shows palatine and ophthalmic branches of seventh, maxillary 

 brancli of fifth, investing mass, rectus externus, and other eye muscles. 



Fig. 29 shows sixtii nerve on left side, rectus externus, and palatine 

 branch of seventh on right side. 



Fig. 30 shows anterior portion of auditory vesicles, seventh and audi- 

 tory nerves, investing mass, and roots of sixth nerve arising from the 

 hind brain. 



Fig. 31 shows roots of sixth nerve on larger scale. X 140. The 

 round shape of some of the cells of these roots is due to their being cut 

 transversely ; there are no ganglion cells. 

 Figs. 32 to 40. — Longitudinal and vertical sections through an embryo of 

 stage 0. X 20 



Fig. 32 shows olfactory nerve and pit, ophthalmic branch of fifth, oph- 

 thalmic, palatine, and mandibular branches of seventh, rectus externus, 

 rectus superior, and obliquus i ferior. 



Fig. 33 shows ophthalmic branches of fifth and seventh nerves, re- 

 mains of first head cavity, and portions of rectus superior, rectus internus, 

 rectus inferior, rectus externus, obliquus superior, and obliquus inferior 

 muscles. 



Fig. 34 shows same parts as preceding figure, at slightly deeper level ; 

 together witii anterior branch of third. 



Fig. 35 shows same parts at still deeper level. 



Fig. 36 shows first head cavity, anterior branch of third, anterior and 

 posterior roots of fifth, root of seventh, auditory nerves, and rectus 

 externus muscle. 



Fig. 37 shows first head cavity, tliird nerve, and junction of sixth 

 nerve, with rectus externus muscle. 



Fig. 3S shows third and sixth nerves, investing mass, and lateral di- 

 latation of pituitary involution. 



Fig. 39 shows sixth nerve from preceding section on a larger scale, 

 X 56. 



Fig. 40 shows anterior and posterior roots of third nerve, pineal body, 

 infuudibulum, aud pituitary iavolution. 



