tactic response here considered Euglena does not react by a number of indiscriminate movements until the right one is accidentally hit upon, but by a direct reflex whose effect is to bring the organism more nearly parallel to the direction of the rays. The phototaxis of Euglena is not so manifestly the outcome of the trial and error method as that of the earthworm. In the latter case light does not cause directly a movement which makes for orientation. The direct response may or may not have that effect. The successful movement is accidentally hit upon, but one can scarcely say this in the case of Euglena in which the orientation takes place more nearly in accordance with the usual 'scheme.

It is perhaps difficult to decide where best to draw the line as regards the employment of the expression trial and error. If it is extended to include the phototaxis of Euglena and other protozoa where there is a gradual adjustment of the path by appropriate direct responses until it coincides with the direction of the rays, we can hardly stop short of including, at least to a considerable degree, the cases of phototaxis that take place according to the commonly accepted theory. We may regard all departures from the straight and narrow path as errors according to whatever theory of phototaxis we may choose to adopt, and we can look upon all movements in that path as successful experiments. I would suggest that if the term trial and error is widened, as seems desirable, so as to include such reactions as are described in the first part of this paper where there is no discernible element of learning involved, its application be limited to those cases in which the adapted movements may be regarded as chance successes. This would exclude the tropisms of the orthodox kind; it would exclude the gradual orientation of such forms as Euglena where oblique stimulation causes a direct response which brings the body more nearly parallel to the rays. It would include many of the reactions of the protozoa where, as in the phototaxis of the blue Stentor, the right direction of movement is hit upon by chance, and a large part of the actions of higher forms. All organisms make errors. In some cases these errors are rectified by an appropriate direct reflex,

in others by the chance success of a random movement. There will doubtless occur many cases difficult to classify where trials are not perfectly random movements but where the stimulus may have a certain directive effect which is in large measure obscured. A tropism of the direct sort is not necessarily a perfectly fixed and rigid affair. It may be a tendency more or les obscured by a lot of random movements arising from internal causes. An organism may be drawn to a certain point through a direct orienting reflex, but if there be at the same time a large element of random activity in its behavior it may seem to reach that point by the method of trial and error. In the trial and error method the random character of the movements impresses us most; in the tropisms, the element of direct determination by the environment. Both of these factors run

through the behavior of all animals, various proportions in different forms.

but they are mingled in

In the lives of most, if

not all, animals both are essential elements in the adjustment of the organism to its conditions of existence.

University of Michigan, Ann Arbor, Mich., Dec. 6, 1904.

NOTES ON THE DEVELOPMENT OF THE SYMPA

THETIC NERVOUS SYSTEM IN THE

COMMON TOAD.1

By WALTER C. JONES, M. D.

With twelve figures.

The work which forms the basis of this paper was done in 1898-1900, at the Zoological Laboratory of Northwestern University, Evanston, Illinois, and was presented in part fulfillment for the degree of Master of Arts. Circumstances prevented the immediate preparation of the results for publication. In the considerable interval to the present, only one paper, as far as I know, has been published, dealing with the development of the sympathetic nervous system; this paper, by HOFFMANN, 1902, is briefly noticed in my review of the literature. The writer wishes to thank Professor WM. A. LocY, Director of the Laboratory, for invaluable help in the work and in the revision of the manuscript.

The earlier view in regard to the origin of the sympathetic nervous system was that advanced by REMAK, to the effect that it arose in situ from the mesoblast. BALFOUR, after his researches on elasmobranch fishes (78), brought forward a new view, namely, that the sym pathetic nervous system arises from the epiblast in connection with the spinal and with certain of the cranial nerves. He claimed that the sympathetic ganglia of the trunk region "are at first simply swellings on the main branches of the spinal nerves." Subsequently, these swellings are removed each from its respective nerve, retaining, however, fibrous connections with the nerve through a short branch, which forms a ramus communicans. They appear at first to be independent, becoming united later by commissures, and forming a continuous cord on either side.

1 Contribution from the Zoological Laboratory of Northwestern University, WILLIAM A. Locy, Director.

SCHENK and BIRDSELL published, in 1879, the results of their observations on certain mammalia, which, as BALFOUR says, "seem to indicate that the main parts of the sympathetic system arise in continuity with the posterior spinal ganglia; they also show that in the neck and other parts the sympathetic cords arise as a continuous ganglionic chain." ONODI ('86), working on elasmobranchs, agrees essentially with BALFOUR, and gives excellently clear figures intended to show that the cells which form the sympathetic cord arise as outgrowths from the spinal ganglia.

PATERSON, in 1891, revived the idea of the mesoblastic origin of the sympathetic nervous system. According to his researches on mouse, rat, and human embryos, the earliest traces of the sympathetic are seen as a cellular cord lying in the mesoblast between the aorta and the cardinal vein, in the anterior dorsal region. This cord is bilaterally symmetrical, and is composed of cells which he claims are differentiated mesoblastic cells. At the time of its appearance, the cord has no connection whatever with the spinal nerves nor ganglia, and constitutes the anlage of the sympathetic. The next step is the formation of the rami communicantes, which arise as fibrous outgrowths passing from the spinal nerves to the sympathetic cord. The ganglia appear next, and are formed at the points where the rami join the cord, resulting presumably from the growth of both sympathetic cells and nerve fibers. The collateral sympathetic is developed by the outgrowth from the sympathetic cord of cellular branches, which later give rise to the ganglia, nerves, and plexuses. In this category are placed the cervical and sacral portions of the sympathetic chain and also rather doubtfully, the grey rami communicantes. This view is repeated by PATERSON, in 1903, in Cunningham's Anatomy.

MARSHALL (93), on the other hand, agrees very closely with the theory advanced by BALFOUR. In frog and chick embryos, MARSHALL finds that the sympatetic nervous system arises "as a series of outgrowths from certain of the cranial and from all of the spinal nerves. These develop ganglionic swellings,"" which later become connected by fibrous commissures, thus forming the gangliated chain of the adult.

HIS, Jr. (97), tracing in the chick the history of an anlage similar to that described by PATERSON, finds, in very early stages, that the cells forming it come from the ganglia of the spinal nerves, thus confirming

Comparative Embryology." Vol. 2, p. 384.

2 Vertebrate Embryology." Page 134.

the researches of ONODI ('86), on elasmobranchs. In the human embryo, His, Jr. finds that the development of the sympathetic begins (in a 10 mm. embryo) with the outgrowth of the white rami communicantes from the spinal nerves. A little later, the sympathetic cord appears and is joined by the rami. These findings contrast with those of PATERSON ('91), who observed that the structure first to appear is the sympathetic cord, the rami developing later.

HOFFMANN'S observations on the sympathetic system of selachians ('99) agree, as far as essential features of development are concerned, with those of BALFOUR and ONODI. Like them, he shows that the cells forming the sympathetic anlage arise in connection with the spinal nerves, thus favoring the view that the sympathetic is epiblastic in origin. In urodeles, he finds ('02) the first trace of the sympathetic occurring as scattered cells connected by slender rami to the ventral branches of the spinal nerves. He does not express an opinion as to whether these cells are of epiblastic or mesoblastic origin. In 30 mm. salamanders, the sympathetic has come to be a continuous chain, in some places fibrous, in other places cellular in character, extending from the first spinal nerve back to the tail region and connected by rami to all of the spinal nerves.

Another question relates to the connection, during embryonic stages, between the sympathetic anlage and the adrenals. These two structures come into very intimate relation with each other during their development. In reptiles, there arises on each side of the vena cava soon after its formation a longitudinal cord of cells, which MINOT calls the "mesenchymal anlage" of the adrenals. On the dorsal side of this anlage and somewhat toward the median line, appear clusters of cells, which are derived from the sympathetic ganglia. They constitute the "sympathetic anlage" of the adrenals.

These two portions come in contact, and at first, in amniota at least, the sympathetic part grows more rapidly, and partially surrounds the mesenchymal portion; but soon the relations become reversed, and the mesenchymal portion gradually surrounds the sympathetic; what finally becomes of the latter is not known. BALFOUR (78) found in elasmobranchs, in the case of the posterior adrenals, that each of these bodies shows a small sympathetic ganglion attached to either end of it, the whole structure being attached to a spinal nerve by a ramus. This mass of cells gradually becomes divided into a ganglionic and a glandular portion; the latter acquiring a mesoblastic investment becomes adrenal, while the former develops into sympathetic tissue. HOFF"Human Embryology." Pages 485 and 486.