These experiments show, then, that there are changes in rate which take place during the incubation period and that the direction of the curve expressing them changes in an orderly fashion. No attempt has yet been made to ascertain the influences operative in bringing about these alterations. It is our intention, however, to attempt to correlate them with such processes as changes in the rate of the general metabolism of the body, changes in structure of the heart muscle, and if possible with changes in the inherent dynamics of the muscle. It will be seen that the number of experiments at a given age is relatively speaking small. The temperature in spite of the precaution mentioned has likewise not been uniform, the range in the greatest number (38) of experiments having been 38°C. ± 1°. A few (four) counts have been made when the temperature was as low as 33.6°C. and a few (two) also at a temperature as high as 40°C. Although a curve as smooth as the one which has been drawn in all probability represents the general course of heart rate change, an explanation of the deviations which have been found is desirable. At least two influences which may be operative suggest themselves. First may be considered the effect of the unavoidable injury incident to cutting a window in the shell and removing so much of the shell membrane as to make the object visible. It has been a uniform practise to discard those eggs in which hemorrhage took place, so that if opening the egg has a share in the difficulty, this operation may be thought of as exerting a mechanical stimulus on a nervous or other controlling mechanism. But whether, especially at the early ages, a mechanism of such a nature exists within the embryo, we do not know. The search for one lies for the present beyond the range of our interest. A second influence responsible for the relatively large deviation in rate from the average may be that of fluctuations in temperature. The air of the room in which the observations were made was, in a given situation, fairly uniform at 38°C. ± 1.0°. as the temperature records show. In the earlier experiments the temperature was read at a distance about 1 foot away from the egg. Between this point and the egg a difference unknown to us may well have existed. It is improbable, however, that it can have been more than 2°C., that is to say, 38° ± 2°. Roughly if the change in rate is approximately 15 beats per degree a maximum deviation of 30 beats may be expected. In point of fact a deviation as great as this did not take place. In later experiments the temperature was read in the immediate vicinity of the eggs, so that a correction can be made more accurately. There is in all probability a high degree of uniformity in the rate of the heart of the chicken embryo at each stage of development, although variation is also found. What the extent of the deviation is that may properly be encountered we do not yet know. It is desirable to pursue this subject further. Observations will accordingly be made and will be reported later.