462 rROCEEDIXGS OF THE ACADEMY OF [Jwly, 



one or more layers of cubical or columnar ciliated cells. The 

 ventral passages, v. p., are lined Avith similar cells, except that no 

 cilia could be made out. On the left is seen, dorsal to the left 

 ventral passage, the tubular organ of Jacobson, j., which is cut 

 anterior to its opening into the ventral passage, v.jy; its cavity is 

 small, in cross section, and only extends for a short distance ante- 

 riorly, the greater part of the organ being a solid rod of cells with- 

 out any visible cavity. On the right side of the figure, which, it 

 will be remembered, is posterior to the plane of the left side, is 

 seen the opening of Jacobson' s organ, J., into the right ventral 

 canal, v. p. The united cavities of the ventral canal and Jacob- 

 son's organ have a sharply triangular outline, which is maintained 

 for a considerable distance posterior to the point at which they 

 first come together. The walls of the organ are of about the same 

 structure as those of the ventral passages. For the sake of sim- 

 plicity the mesoblast cells in this and in all of the preceding sec- 

 tions have not been represented. They are typical mesoblast cells 

 and surround numerous blood vessels. 



Fig. 5a represents a sagittal section of an embryo of the same 

 stage of development as the one represented in fig. la. The sec- 

 tion is nearly, but not exactly, in the median plane, so that some of 

 the organs are cut medianally while others are cut to one side of 

 the median plane The general outline of the head is well shown 

 and the relative positions of the main regions can be seen. The 

 brain, br., and spinal cord, s.c, are represented in the heavier 

 shading; the cartilaginous parts, including the vertebral column, 

 V.C., in the lighter areas. The great size of the nasal cavity is due 

 to the fact that the section passes through one of the main nasal 

 passages in the plane of its greatest diameter, fig. 2/. The 

 other parts of the head will be easily understood by reference to 

 the letters. As in the previous sections, the mesoblast has been 

 omitted for convenience and simplicity. 



Letter I XG of Figures. 



b. — Basal plate of cartilage. 



b.c. — Cartilage around the brain. 



hr. — Brain. 



h.w. — Body wall. 



c. — Later.il part of nasal canal. 



d. — Diverticulum of uasal canal. 



e. — Eye. 



7i. — Hypophysis. 



j. — Jacobson's organ. 



L — Lens. 



l.j. — Lower jaw. 



I. /I.e. — Left uasal canal. 



l.n.g. — Left nasal gland. 



7/i. — Muscle of the eye. 



