from what will be the anterior portion of the roof of the mouth, and the neural tube, a small button of cells (fig. 1). No limiting membrane was made out between this group of cells and the brain wall. The cells were like those of what may be termed the unpaired olfactory placode into which they grade.

In a stage six hours older which has not been figured, the button appears as a cord of cells tapering downward and forward

neural tube.

unpaired placode

sucking disk⋅

paired olfactory placode

FIG. 1. Transverse section (slanting slightly upward and backward) of the anterior end of an embryo of Amia forty hours before hatching. Shows the paired and unpaired nasal placodes, the anterior end of the neural tube, and the sucking disk. Yolk granules are still evident in sucking disk. Iron hæmatoxylin and acid fuchsin, 6 micra thick. 215.

from beneath the anterior end of the neural tube to the unpaired portion of the olfactory placode. A similar unpaired portion of the olfactory placode has been described by von Kupffer ('93) in the sturgeon. From this fact and the unpaired condition in Amphioxus and the Cyclostomes he argues for primitive monorhinism in the vertebrates and secondary amphirhinism. The

cord of cells just described in Amia would seem to correspond in position to the place where von Kupffer's lobus olfactorius impar ('93, fig. 15) comes into contact with his mediane Riechplatte. It was at first suggested to my mind that in Amia the cord of cells. might be the ectodermal portion of the hypophysis, but a mass of cells believed to be Rathke's pouch is already present near the optic chiasm. Johnston ('05, p. 202) thinks the lobus olfactorius impar marks the dorsal border of the neuropore. As the first evidences of the nervus terminalis appear much later, no stages of Amia were cut at a date early enough to show the neuropore. Just as this goes to press Johnston ('09) publishes an article describing nerual crest near the olfactory placodes in lower vertebrates. The early development of the unpaired olfactory placode and of the neuropore should be investigated fully in Amia.

At about the middle of their antero-posterior extent the olfactory pits come into contact with the brain wall forty hours before hatching, when there is still an unpaired placode. This point of contact is some eight sections posterior to the anterior end of the neural tube, that is to say, about fifty micra. The connection is very slender, as it is comprised within two sections. It cannot be definitely stated that any fibers extend across from the placode to the brain at this time, but there is solution of the limiting membrane of the neural tube at this point. There is a delicate limiting membrane, shown by the fuchsin stain producing a red line, between the mesodermal elements and the caudodorsal part of the olfactory placodes, but none could be made out anteriorly and ventrally. The lack of a limiting membrane about the anterior ventral part of the olfactory placodes in the early stages was thought to be due to the recent connection of these placodes with the unpaired placode (fig. 1), and that no membrane has as yet formed in this position. Perhaps this may be the reason why olfactory fibers were observed to appear first at the anterior end of the placode, in most cases, when the olfactory nerve develops.

The condition of the olfactory placodes at a time six hours later than the stage described above, does not differ greatly and has

not been figured. At this time, which is thirty-four hours before hatching each of the paired placodes is still connected with the ectoderm at a point between the sucking disks, although by a more slender cord of cells than was the case in the earlier stage described. But the points where these two cords of cells reach the ectoderm adjacent to and between the two halves of the sucking disk, are now removed farther laterally from each other. From a position midway between the two halves of these two paired placodal connections with the roof of the mouth, there is still a strand of cells extending from the ectoderm to the anterior ventral edge of the tip of the neural tube. This cord of cells is largest near the neural tube. There is no appreciable increase in the size of the connection of the paired placodes with the brain over the previous stage described. A slight indentation of the cuticular ectoderm shows where the external opening of the nasal sac will appear later. The embryos showed slight motions at this age.

About twelve hours later than the last stage, or some twentytwo hours before hatching, there is no unpaired nasal placode nor any evidence of a cord of cells from the roof of the mouth to the brain wall at its anterior end. The paired placodes in one or two preparations of this stage have shown a slight elongation into a point forward just beneath the ectoderm. The nasal placodes, as well as the anterior end of the neural tube, are farther removed from the anterior end of the snout than was the case earlier. At this time there appears to be a fibrous connection between the olfactory capsules and the brain (fig. 2). Whether this connection is protoplasmic or is composed of true neurofibrils such as those described by Paton ('07), was not determined, as the method of Bielschowsky which was tried, did not differentiate any fibers in young or embryonic Amia. The connection is quite slender, as it is seen in only two sections, and is in consequence not more than twelve micra in thickness. A very few cells of the placode near the place of origin of the fibers are slightly larger than the others at this age. The distance from the placode to the outer brain wall is almost nothing at this age and there are no nuclei in the course of the olfactory nerve.

There is a definite membrane about the nasal capsule except where the olfactory nerve originates and for a slight distance anterior and ventral therefrom.

At a period three hours later, or about nineteen hours before hatching, a slight increase has taken place in the number of large cells in the placode near the origin of the olfactory fibers. The latter are not numerous, being confined to three or four sections

FIG. 2 Transverse section of a embryo Amia 22 hours before hatching. Shows contact of the paired olfactory placode with the neural tube. Preparation as above. X 450.

six micra thick. Some cells which have the characters of neuroblasts are now seen within the brain wall near the point where the brain has raised slightly to meet the olfactory nerve. This is the beginning of the olfactory bulbs. There are mitoses in the layer of germinative cells next the brain ventricle. Also, mitotic figures are seen here and there throughout the nasal placodes which have no lumen as yet. A slightly greater space exists at

this stage, than formerly, between the placode and the outer wall of the brain. Mesenchyme accompanied by capillaries and blood corpuscles has pushed into this space. An outer zone of nerve fibers has appeared just inside the brain wall and extends from a point anterior of the olfactory nerve to some distance posterior of it.

Three hours later there are many more fibers in the olfactory nerve but no nuclei in its course. In one preparation of this stage a very small cavity can be made out near the center of the placode. This cavity is not connected with the outside, but lies a little nearer the lateral side of the placode opposite the slight depression already mentioned as occurring in the epidermal layer of the ectoderm. The cells of the mesial side of the placode have assumed their columnar epithelial shape with nuclei at some distance from the lumen, as in the adult; but those on the opposite side of the lumen, where the external opening will appear later, are more rounded and irregular in arrangement. From this stage onward it is readily seen that the nasal capsule is continuous at its border just under the epidermis with the inner layer of the ectoderm in which taste-buds arise a little later. It can be said then, that at this age, some sixteen hours before hatching, the cells on the median side of the placode have assumed a more or less definite character of neuro-epithelium and it is probable that the olfactory nerve is composed of definite neurofibrils. The fuchsin stain shows a plainly fibrillar structure.

Other stages are about the same until six or seven hours before hatching, when in horizontal sections two bundles of fibers can be seen converging to a union just before entering the brain wall (fig. 3). The anterior one is slightly larger and is probably the older, as the earliest fibers are generally seen near the anterior border of the placode. Some three or four cells are seen among the olfactory fibers just as they leave the nasal capsule. Mesenchyme cells having capillaries and blood corpuscles in their midst, crowd near the nerve but in many of the preparations can be distinguished by their staining reactions from the cells inside the nerve. The two branches of the nerve can be followed well in among the nuclei of the nasal capsule. There is no external