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The use of dissected pig embryos for classwork in embryology suggested itself to the writer some years ago. Upon trial it was found that very instructive preparations could be made and with much greater ease than might be expected. It is difficult for students to grasp the relations of developing organs as seen in sections and a dissected embryo showing the primitive organs in position is very helpful in remedying this evil. It is the intention of the author to present, in a future paper, the results of his work along these lines, with directions for dissection and figures of the more instructive preparations. The form and relations of the various organs may be seen as accurately as in reconstructions made from serial sections by experts. There is this disadvantage in dissection, that some of the finer details of structure may be lost. For research it commends itself as a check to errors which may occur in making reconstructions; for it enables one in a short time to study a considerable number of embryos.

The nervous system lends itself most easily to dissection. The mesenchyma crumbles away from the more tenuous nervous tissue of suitably prepared embryos, making it possible to lay bare the entire nervous system of pigs varying in length from 6 to 20 mm. The smaller embryos are more easily dissected, as no cartilage or bone is encountered.

My dissections brought out some points in connection with the cerebral nerves which have not hitherto been cleared up, and my

THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO. 4.

purpose in the present paper is to describe the rudimentary ganglia which occur between the first cervical nerve and the vagus, ganglia which I shall refer to as the hypoglossal ganglia because of their undoubted connection with this nerve. My descriptions will necessarily deal with the development of the last four cranial nerves, the glosso-pharyngeal, the vagus, the spinal accessory, and the hypoglossal.

LITERATURE

The occurrence of hypoglossal ganglia was first described by Froriep ('82) in the sheep and the ox. He traced the development of a single ganglion, anterior to the first cervical, which it resembled in form, though smaller in size. The single distal root of this ganglion joined the most caudad root of the hypoglossal nerve. Anterior to this hypoglossal ganglion the neural crest was undifferentiated. Froriep and Beck ('95) found a precervical ganglion present in the adult throughout those groups of mammals in which the first cervical ganglion was well developed.

Martin ('91), investigating cat embryos, found five ganglionic masses posterior to the jugular ganglion, and five roots of origin for the hypoglossal nerve. He concludes, therefore, that these ganglia are the dorsal ganglia of the hypoglossal, though he gives no figures in support of his view.

Lewis ('03) in his excellent paper on the anatomy of a 12 mm. pig found extending caudad from the jugular ganglion "a beaded commissure ending in a small knob. In the track of the commissure, but separated from it, is an irregular ganglionic mass. After another interval there appears a small fragment, then follows the first cervical ganglion." In one case he found a small fiber bundle connecting the irregular ganglionic mass (Froriep's ganglion) with the hypoglossal nerve, but considers its "relation with the commissure" as "far more striking than its resemblance with a spinal ganglion." He finds the ganglion "connected with the commissure in pigs of 17 mm." In a dissected pig of the same length "the hypoglossal ganglion appeared as a detached part of

the ganglionic chain running forward to the vagus. This commissure could not be subdivided into definite ganglia; it was characterized by irregular swellings and spurs."

Streeter ('04) in tracing the development of the peripheral nerves in human embryos finds a ganglionic crest extending from the first cervical to the superior ganglion of the glossopharyngeal and partly ensheathing the fibers of the spinal accessory nerve. In embryos 10 to 13 mm. long the neural crest becomes differentiated into four or five rather diffuse cell masses. Froriep's ganglion resembles the others, being irregular in form and without roots. The hypoglossal nerve originates as four or five parallel roots. There is no correspondence between these and the rudimentary ganglia, nor are the ganglia segmentally arranged. He considers the three or four anterior cell masses as cerebral ganglia and "not to be confused with the precervical ganglion of Froriep."

MATERIALS AND METHODS

The number and length of the embryos dissected are given in the following table:

All drawings were made with the aid of an Abbe camera lucida and a Zeiss a* objective. The embryos were fixed in Zenker's fluid and the dissections were first stained, cleared in creosote and drawn as transparent objects. It was thus possible to locate microscopic cell masses and trace the course of very small fiber bundles. The dissection was then transferred to alcohol and examined as

an opaque object by reflected light to obtain the contour of the different structures. By making several dissections of the same stage I believe that the finer structures were more accurately and completely reproduced than could be done by serial reconstructions.

DESCRIPTION OF DISSECTIONS

Stage 1. 6 mm. In this embryo (fig. 1) the ganglia were connected from the glossopharyngeal (IX) back to the caudal region

by continuous bands or loops of cells, undifferentiated portions of the neural crest. The ventral roots of the spinal nerves (C1 C2) are large, but the dorsal ganglia show little differentiation into fibers, though short distal and proximal roots are present. The hypoglossal originates as five or six parallel roots, resembling those of the spinal series but uniting, in the later embryos of this stage, to form a common trunk (XII). The spinal accessory (XI), as an arched bundle of fibers, could be traced from the fourth cervical ganglion cephalad to the vagus. Dorsal to the accessory and partly ensheathing it is a flattened band of cells, the

neural crest (crist. neur.), extending forward to the jugular ganglion of the vagus (gang. jugul.). Opposite the posterior root of the hypoglossal a marked ventral loop and thickening in the crest (gang. Froriep) shows the position of Froriep's ganglion. Anteriorly the crest of cells is broader and a few short proximal rootlets are present. A depression separates it more or less completely from the cells of the jugular ganglion which is flattened and diffuse with 8-10 short proximal roots. The glossopharyngeal is short and its superior ganglion is joined to the jugular ganglion by a small cord of cells.

The remarkable features at this stage are then: the early development of the spinal roots; the resemblance of the hypoglossal to a series of ventral spinal roots; the existence of a nearly undifferentiated neural crest between the jugular and the first cervical ganglion.

Stage 2. 8.5-9 mm. In embryos of this stage (figs. 2 and 3), the roots of the spinal nerves are longer and more fibers are developed. The first cervical ganglion is distinctly double in fig. 2. It is still connected with the second cervical ganglion by a loop of cells. The neural crest between the first cervical and the jugular ganglia shows the most marked change. The slight enlargement opposite the posterior root of the hypoglossal which we saw in the first stage has now grown to be a spindle-shaped mass of cells (gang. Froriep) with two proximal roots and a distal bundle of fibers which extends to the root of the hypoglossal. This ganglion (Froriep's) is still connected with cellular loops (nod.) of the neural crest, but in this respect it does not differ from the cervical and sacral ganglia of this stage. It strongly resembles one of the two cell masses composing the first cervical ganglion. Anterior to it is a smaller mass of cells (gang. hypogl.) from which a proximal root is developing. This is the "terminal knob" of the "commissure" figured by Lewis in the 12 mm. pig (1903, pl. I). Anteriorly the crest shows five diffuse irregular cell masses which become gradually larger toward the jugular ganglion with indications of proximal roots. The jugular ganglion is of more definite form and is pointed ventrally. Dorsal