tions of the Limulus embryos was shortly before the time of hatching. At this stage the embryo has a distinctly limuloid appearance in nearly all respects. Ail the appendages are formed, and are movable, with the exception of the long telson characteristic of the animal in later stages. The legs are formed on the same plan as those of the adult female, the secondary sexual modifications of the chelae of the first pair of walking legs in the male, not yet appearing. In this and the succeeding stages the embryos have a general, superficial resemblance to a trilobite which has led to the designation of these as the "trilobite stages" in the development. The embryo lies in the "vicarious chorion" (cf. PACKARD '72, and KINGSLEY, '85, p. 525) surrounded by fluid. The "vicarious chorion" is considerably greater in diameter than any dimension of the embryo, so that there is considerable free space on all sides of the latter. The embryo at this stage is about 4 mm. in length. The following account of the behavior within the "vicarious chorion" applies to embryos at any time within a week before hatching. Closer time relations than this, as will appear from the account, cannot be fixed in the development of the reactions of this organism.

Normal Position of the Embryo.-The embryo lies at the bottom of the hollow sphere formed by the "vicarious chorion," with its neural side uppermost. This position is simply the result of the action of gravitation, the embryo sinking to the bottom of the sphere because of the fact that its specific gravity is greater than that of the surrounding fluid. The reason for its lying with the neural surface uppermost is to be found in the fact (to be brought out in detail in another connection) that it is unable, under the circumstances in which it finds itself, to get into and retain any other position in which it is in stable equilibrium.

Movements within the "Vicarious Chorion."-In the descrip

I shall speak of the developing organisms throughout as "embryos." The rupturing of the "vicarious chorion" and beginning of free larval life, will be termed the "hatching." These expressions are used merely for verbal con. venience.

tion of the movements of the embryo within the "vicarious chorion" the abdominal appendages will be considered first, as the phenomena here are relatively simple in character.

The abdominal appendages (operculum and gills) begin characteristic, rhythmical respiratory movements at least five days before hatching. It is probable that in reality such movements begin even earlier than this, but I have no observations going farther back. The ordinary respiratory movements when first observed are precisely like the same movements in the adult Limulus. They consist of a rhythmical, up-and-down beating of the gills, each gill book being opened during the phase of expansion, or "inspiratory" phase, to adopt the terminology of Miss HYDE ('94).

There is, however, one significant difference in the respiratory movements of embryos and adults. This is in the rate. In the adult the normal rate is about twenty-five to thirty beats per minute. In the embryos the rate is markedly more rapid, the average number of beats from my observations being sixty per minute. The range of variation is from 55 to 60 beats per minute. The rhythm of the beats is quite as per

fect in the embryos as in the adults.

I

These respiratory movements are the only movements which the abdominal appendages perform before the embryo leaves the "vicarious chorion," so far as I have observed. was never able to detect any tendency towards swimming movements of the gills before the time of hatching, although the embryos were under observation for six or more hours every day, and especial attention was paid to this point. This absence of swimming movements is rather remarkable in view of the fact that all the embryos begin swimming immediately after hatching.

In addition to the swimming movements the complex "gill-scraping" reflexes are absent, according to my observations, in embryos prior to the time of hatching. Certain of these reflexes have been described by Miss HYDE (1. c. p. 432, and Fig. 3). There also occur in adult Limuli, under certain conditions, complex gill-scraping movements of the sixth legs. I

have never seen these in the embryos before the time of hatching.

The intervals of rest in the respiratory movements, which in the adult occur frequently and may last for an hour at a time, are much less frequent in their occurrence in the embryos, and do not continue for such long periods.

For some time before hatching the thoracic appendages can be seen to be making active movements almost continuously. These movements vary greatly in force in different individuals, and at different times with the same individual. When they first appear they are usually very weak and increase in violence as development proceeds. At first it was thought that these movements were entirely random and aimless in character. They appeared to consist entirely of mere wavings and stretchings of the legs. Closer observation showed, however, that they were rather more definite than at first appeared. It was seen that they were the cause of the curious translatory movements of the egg as a whole. As the time of hatching approaches one notices very frequently that a particular egg lying by itself on the level, smooth bottom of a glass dish will suddenly begin to move, and slowly roll along the bottom, usually in a straight line. Frequently an egg will roll several centimeters in this way, although usually it does not cover more than from one to two centimeters.

These translatory movements of the egg as a whole are caused by the movements of the thoracic appendages in the following way. As the animal lies in the bottom of the "vicarious chorion," in the manner already described, the anterior and lateral margins of the cephalothorax are in fairly close contact with the inner surface of the "vicarious chorion."

stage the chelae of the walking legs end in very sharp points. As a result of these two conditions, when the legs are extended the points of the chelae catch on the inside of the "vicarious chorion.' The legs are usually directed somewhat forward when they are extended, and as a consequence of this and of the normal anatomical position of the legs it follows that the

chelae catch at points lying on the anterior' hemisphere of the vicarious chorion." Evidently then if the legs are extended, or in other words, the chelae are strongly pushed away from the body, when they are caught in the "vicarious chorion," a movement of the embryo or of the "vicarious chorion" will be caused. Which of the two shall move, will depend on the circumstances. If the "vicarious chorion" is held in any way, the embryo is moved within it, in the following manner. The anterior edge of the cephalothorax is pushed more and more towards the lowest part of the hollow sphere in which the embryo lies, until finally the whole embryo is nearly or quite in a vertical position, resting on the anterior margin of the cephalothorax. If the legs still continue their kicking against the inside of the "vicarious chorion" the embryo is completely turned over and falls back to the lowest position of the sphere, with the haemal side uppermost.

On the other hand, if the "vicarious chorion" is not held in any way, the same action which in the previous case caused the embryo to turn over within it, causes the egg as a whole to roll slowly over the substrate. That this must be the case is evident if it is remembered that the weight of the embryo tends to maintain the latter in a constant position with reference to the center of the earth, while the hollow sphere, against the inside of which it is pushing, is rotated about it. On account of the fact that the embryo lies in a fluid in the "vicarious chorion" there is practically no internal friction to hinder this rotation of the sphere about it. It will be seen that, from the point of view of mechanics, whether the "vicarious chorion" as a whole shall rotate or not, depends on the relation which happens at the time to prevail between the weight of the embryo and the external forces hindering any movement of the sphere. If the weight of the embryo overbalances the external forces the sphere will rotate, while on the other hand, if the external

1 Of course the terms anterior and posterior are here used to refer to the portions of the "vicarious chorion" nearest, at any given time, to the anterior and posterior ends of the embryo.

hindrances are great enough to overbalance the weight of the embryo, the latter will turn over within the sphere.

In the movements of translation of the eggs as a whole the direction of the movement is usually such as to keep the abdomen of the embryo in advance. This is a mechanically necessary result if the movements are produced in the manner just described. Of course, circumstances will sometimes prevent the egg from moving in a perfectly straight line, and again, the pressures exercised by the legs on the two sides of the body are not equal in amount. That they are about equal in the long run is shown by the fact that in the great majority of cases when free movement of the egg for some distance occurs, this movement is very approximately in a straight line. In cases where the movement is somewhat irregular the longitudinal axis of the embryo may not lie exactly in the path of motion, yet in no case have I ever seen the anterior end of the embryo in advance in the free translatory movements of the egg. The movements are always either somewhat sidewise with reference to the embryo, or, much more commonly, with the abdomen in advance.

In the behavior just described there is no definite co-ordi nation in the movements of the legs. Each leg acts by itself, and there is no rhythmic order in which the different legs move, as is the case, for example, in the gustatory reflexes of the adult Limulus (cf. PATTEN, '93, pp. 7-9).

The behavior when the "vicarious chorion" is held so that it cannot move offers some points of interest. The embryo is seemingly unable to maintain itself with the haemal side uppermost even after it attains that position in the manner described above. This is apparently a consequence of the fact that the organism after getting into the upright position does not stop the movements of the legs. When the legs are extended with the animal in an upright position the whole body is of course raised till its haemal surface is nearly in contact with the uppermost part of the sphere. When the embryo is in this position, if the effective pressure of the legs becomes greater on one side than on the other, as invariably happens, it forthwith