(From the Anatomical Laboratory, St. Louis University.)

With Plate V.

A superficial study of the optic vesicles as they develop in Necturus showed certain interesting features, which have not been heretofore described. On account of the large size of its cellular elements, the cytological changes in this animal admit of a more exact interpretation than is afforded by other vertebrates.

The formation of the neural plate in Necturus is initiated by certain changes in the ectodermal cells, which elongate from the low cuboidal of the superficial ectoderm to the long columThe same changes

nar type, as shown in Fig. 1, e.c and n.c. are shown under higher magnification in Figs. 7 and 8, Fig. 7 representing a cell near the center of the neural plate, and Fig. 8 an ectodermal cell beyond the margin of the neural plate. The cells of which Fig. 7 is a type are divisible into two parts, a proximal and a distal. The former is full of yolk granules and usually contains the nucleus. This nucleus varies in shape from round to long oval, its long diameter being from two to three times the short. Sometimes it is irregular in

shape, in which case it is usually found near the center of the neural plate. When the nucleus lies in the distal part of the cell, it is frequently observed undergoing mitosis.

The distal part of the cell may contain a few yolk granules, which are usually small and of indistinct contour, as shown in Fig. 7. This portion does not stain readily, shows a distinct membrane, and is free from yolk granules. Such polarity is most marked in the cells (Fig. 1, n c.) near the center of the neural plate. From here outwards, towards either side of the neural plate, the nuclei are rounder, the cells shorter, and the granules encroach more and more on the clear portion of the cell.

As soon as the neural plate is well defined (Fig. 2, n.p.), the outlines of the individual cells become less distinct, though each presents the same general characteristics observed in the preceding stage, as represented in Fig. 7. The neural plate is now composed of two or three layers of columnar cells. The neural cells forming the upper layer show the portion spoken of above as free from yolk granules. They are so arranged that a clear band stretches from one margin of the plate to the other. The ectoderm of the remainder of the embryo becomes two-layered as it approaches the neural plate, and ends abruptly at the margins of the latter, which already shows an incipient infolding (Fig. 2, n.p.). The further development of the neural plate is characterized by an increase in the number of cells composing it, and by an approximation of its margins, which grow together to form a tube, as shown in Figs. 3, 4 and 5.

By the time the anterior part of the neural canal is formed and the Anlagen of the optic vesicles appear, the neural tube has the general shape of a spherical triangle, the two basal angles being formed by the Anlagen of the optic vesicles (Fig. 3, o.v.). The plane of the section represented is through a part where the canal has not quite closed. The wall is composed of many layers of cells, and varies in thickness in different localities. It is, in general, thinner in the region of the future optic vesicles. The cells forming the lining layer abut upon the lumen in an irregular line, as represented in Fig. 3.

The nuclei (n.n.) in the wall of the neural canal vary greatly in outline. Some are round, some oblong, while a large

nomenon.

number are irregular, showing a prolongation in the direction of the lumen. Some of the latter are shown under higher magnification, in Figs. 9, 10, 1I. These represent three cells from the same field, and illustrate different stages of the same pheThe body of each nucleus shows a number of scattered karyosomes, while the prolongation is more or less striated, the striae running parallel to its long axis. The appearance of the cytoplasm and the nuclei indicates that the cells are migratory.

Some of the nuclei are undergoing karyokinesis and these, as well as the ameboid forms of nuclei, are more numerous where the walls are thinnest, being especially so in the region of the optic vesicles. In places the cells are seen dividing, where one-half is being thrust out into the lumen.

The lumen of the canal contains a mass which can be resolved into three constituents; (1) and most numerous, are round structures slightly larger than the neighboring nuclei, which are massed together and more or less completely fill the lumen. These will be shown to be cells in various stages of degeneration; (2) nuclei and larger remnants of the same; (3) detritus. The first of the contents of the lumen consists of well defined cell outlines. They are round, of nearly constant size, having an average diameter of 40 μ, and are usually clear and devoid of yolk granules, as depicted in Fig. 3, .c, They are more numerous near the head end of the embryo, especially in the region of the future optic vesicles.

The nuclei lying in the lumen are, in a varying degree, smaller and less deeply stained than those in the wall of the tube. They may be round or may assume ameboid forms (Figs. 12, 13, 14). Some are pear shaped (Figs. 13, 14), with the larger mass extending into the lumen. Many appear (Fig. 13) as if the chromatin is passing out of the basal portion of the cell and becoming rounded up in the distal portion.

The round forms of the nuclei either show a cell membrane around them, or where this is not distinct, there are yolk granules present, indicating the position of the cell. A few nuclei lie in the lumen independent of any connections with a cell, but

from their small size, faint staining capacity, and general indistinctness, we may conclude that they are degenerating.

The third constituent of the contents of the lumen is a debris composed of fragments of yolk granules, disintegrating nuclei, and remains of cell membranes.

In the next stage (Fig. 4) the development of the embryo has advanced so far that the optic vesicles (o.v.) are distinct evaginations of the neural canal. The figure shows a section through the neural canal and the centers of the optic vesicles. The nuclei of the neural cells lying next to the lumen show a tendency to assume ameboid forms which however, are not as pronounced as in the preceding stage. They are most numerous in the dorsal wall of the canal, and in the superior wall of the optic vesicles. Along the margin, nuclei undergoing degeneration are occasionally seen.

The center of the lumen is entirely free from cellular elements or their remnants. The remainder of the neural canal shows the cell outlines described above, some nuclei and a few yolk granules. The cells of the optic vesicles are more compact, larger and more irregular than those lying towards the center of the canal. Where the section does not pass exactly through the center of the optic vesicles, but on either side, the central space is larger, and the mass of cell outlines lies in or near the optic vesicles. The nuclei lying in the lumen are round and no longer show ameboid forms. In Fig. 4, there are shown nuclei (l.n.) with well defined chromatin threads, while in other nuclei these threads have fused into a dense mass.

Fig., 5 represents a section through the neural canal and optic vesicles (o.v.) of the next stage. The cells in the wall are arranged in two or three layers. The dorsal wall of the canal is still from three to six layers of cells in thickness, but most of the nuclei are ameboid in shape or undergoing mitosis. In the lumen the conditions described above are accentuated. The clear central space is very large and extends nearly from the dorsal to the ventral wall of the canal. The remainder of the lumen, especially the optic vesicles, is filled with cells and nuclei in various stages of degeneration.