The present lack of segmental arrangement displayed by these ganglia does not preclude their metameric origin. Their development begins considerably later than that of the spinal nerves, and the rapid growth of the region they occupy, before they make their appearance, may cause them to shift their positions with relation to their myotomes. They certainly appear in regular series and their early development is similar to that of the spinal ganglia. As to the number of dorsal ganglia represented in the hypoglossal series, no absolute statement can be made. The evidence of comparative anatomy goes to show that four or five spinal nerves have been added to the cranial series as a result of the union with the cranium of a corresponding number of vertebrae. Meeks ('09) finds in an Acanthias embryo three rudimentary spinal ganglia located between the vagus and the first spinal nerve, the ganglion of which would correspond to Froriep's ganglion in mammals. According to this evidence, four dorsal ganglia have become rudimentary structures in mammals and the corresponding ventral roots have united to form the hypoglossal trunk. Regarding each pair of the eight ganglionic nodules found in the 13 mm. embryo as homologous to a single spinal ganglion, then we would have the same number of ganglia, four, represented between the first cervical and the jugular. The more anterior roots of the hypoglossus, which are found in the early embryos but disappear in the later stages, represent ventral roots of the vagus and glossopharyngeal according to the observations of Bremer ('08). CONCLUSIONS 1. The jugular and superior ganglia of the vagus and glossopharyngeal nerves, the hypoglossal ganglia and ganglia of the spinal nerves arise in the pig embryo from a continuous neural crest, as observed by Streeter in human embryos. 2. The hypoglossal ganglia are retarded in their development, but appear in embryos of 13 mm. as a series of eight connected cell masses of nearly equal size. 3. According to their development, the hypoglossal ganglia can be divided only artificially into a cephalic cerebral group and a caudal pre-cervical group. 4. The first cervical and other spinal ganglia are often of double origin, composed of two spindle-shaped masses, and generally possess two distal roots. 5. The spindle-shaped ganglion of Froriep with its single distal root would therefore represent but one half of a spinal ganglion. 6. The degree of development of the hypoglossal ganglia varies in different embryos; in the same embryo the right side may be better developed than the left, and vice versa. This is good evidence of their rudimentary or vestigial character. 7. One, frequently two or three, and in one case four hypoglossal ganglia possessed single distal roots and the fibers of three of these joined the hypoglossal nerve. 8. The connection of the hypoglossal ganglia with each other and with the jugular ganglion represents a persistence of the neural crest. It is similar to the connections which were found persisting between the roots of the spinal ganglia. 9. If we consider a pair of hypoglossal ganglia as the equivalent of a single spinal ganglion, four such ganglia would be represented in pig embryos, between the jugular and the first cervical. 10. The hypoglossal trunk develops as five or six separate ventral roots, at first parallel and independent, later uniting to form a single nerve. 11. The hypoglossal ganglia reach their maximum development in embryos 17-20 mm. long, then retrograde, though ganglia at both ends of the series may persist in the adult. 12. The spinal accessory nerve develops very early, being well formed in the youngest embryos examined (5 mm. long). As development proceeds the fibers of the spinal accessory root may be recognized farther and farther caudad. In a pig of 17 mm. a few accessory fibers were traced to a point opposite the eighth cervical ganglion. THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO, 4. BIBLIOGRAPHY BREMER, J. L. Aberrant roots and branches of the abducent and hypoglossal 1908. nerves. Jour. Comp. Neur. and Psych., vol. 18, pp. 619-639, 9 figs. FRORIEP, A. 1882. Ueber ein Ganglion des Hypoglossus. Archiv f. Anat. u. Physiol., FRORIEP, A. UND BECK, W. Ueber das Vorkommen dorsaler Hypoglossuswurzeln mit Ganglien in Reihe der Säugethiere. Anat. Anz., Bd. 10, pp. 688-696. 1895. LEWIS, F. T. 1903. The anatomy of a 12 mm. pig. Amer. Jour. Anat., vol. 2, pp. 211225, 4 pls. MARTIN, P. Die Entwickelung der neunten bis zwölften Kopfnerven bei der 1891. Katze. Anat. Anz., Bd. 6. MEEK, A. The encephalomeres and cranial nerves of an embryo of Acanthias vulgaris. Anat. Anz., Bd. 34, pp. 473–475. 1909. STREETER, G. L. The development of the cranial and spinal nerves in the occipi1904. tal region of the human embryo. Amer. Jour. Anat., vol. 4, pp. 83116, 4 pls., 14 text figs. Accepted by the Wistar Institute of Anatomy and Biology, May 16, 1919. EXPLANATION OF FIGURES Printed September 9, 1910 All drawings were made with the aid of a camera lucida and have been reduced to a common magnification of about 25 diameters. The figures show in surface view the right side of the myelencephalon and spinal cord from a point anterior to the origin of the glossopharyngeal nerve to a point just caudad to the first, second or third cervical ganglion. The following abbreviations have been employed: C1, C2, first and second cervical ganglia; crist. neur., neural crest; gang. Froriep, Froriep's ganglion; gang. hypogl., hypoglossal ganglia; gang. jugul., jugular ganglion; gang. nodos., ganglion nodosum; gang. petros., ganglion petrosum gang. sup., superior ganglion; nod., persisting cellular nodules of neural crest; sp. cord, spinal cord; IX, X, XI, XIII, glossopharyngeal, vagus, spinal accessory and hypoglossal nerves; XI', peripheral portion of spinal accessory nerve. The present investigation of the development of the sympathetic nervous system in birds has grown out of an investigation of the development of the sympathetic nervous system in mammals. It was undertaken in order to further exact knowledge concerning the development of the sympathetic nervous system, to extend From the Laboratories of Animal Biology of the State University of Iowa. Prof. Gilbert L. Houser, Director. the writer's observations on the histogenesis of the sympathetic nervous system in mammals, and to point out certain morphogenetic differences in the development of the sympathetic system in birds and in mammals, with a view to their phylogenetic signifi cance. Birds and mammals have become specialized along divergent lines. Their special habits of life have brought about modification in the course of ontogeny as well as in adult structure. The sympathetic nervous system, which is concerned primarily with the control of the purely vegetative functions, has not escaped the modifying influence of specialized habits. It is hoped, therefore, that a more exact knowledge of the development of the sympathetic nervous system in birds may throw some new light on the problems involving the structural and the functional relationships of the sympathetic system to the central nervous system. Inasmuch as the literature bearing on the development of the sympathetic nervous system has been reviewed by the writer in a recent paper,2 only such references will be made to the literature in this paper as seem to be necessary. The observations set forth in the following pages are based on embryos of the chick. The embryos were fixed in chrom-acetoformaldehyde. The sections were cut to a thickness of 10 micra and stained by the iron-hæmatoxylin method. This method, as indicated in the earlier paper, was found best adapted for purposes of this research. OBSERVATIONS 1. Sympathetic trunks (a.) Introductory. His, Jr. ('97) called attention to the fact that in the chick two pairs of sympathetic trunks arise in the course of ontogeny. These he has designated as the "primary" and the "secondary" sympathetic trunks. According to his observations, the primary sympathetic trunks arise about the close of the third 2 The development of the sympathetic nervous system in mammals. Journal of Comparative Neurology and Psychology, vol. 20, no. 3. |