330 GRACE MEDES 



not swim but remained on or near the bottom of the dish the 

 liquid was siphoned off from above as thoroughly as possible, 

 and the blastulae poured into the fresh solutions. Transfers 

 were made at the close of each twenty-four hours of develop- 

 ment, so that during the five days of the experiment nine changes 

 were made: four during the first hour, one after about six hours, 

 and after twenty-four, forty-eight, seventy-two and ninety-six 

 hours respectively. 



All the sea-water employed was obtained from the harbor 

 at a sufficient distance from shore to insure freedom from shore- 

 contamination, conveyed in glass vessels and filtered before 

 using. During the experiments the finger-bowls were kept 

 covered with glass plates to prevent evaporation and kept 

 standing in the aquarium surrounded by running sea-water. 

 From time to time the temperature of this water was compared 

 with that of the open sea at high tide and about three feet be- 

 low" the surface, and at no time did it show a greater difference 

 than one-half of one degree. The greatest possible cleanliness 

 was observed throughout, every dish and instrument being steril- 

 ized each time it was used. 



Rate of growth was determined by measurement of the skele- 

 ton, as this seemed to afford the most nearly constant character 

 and the one most capable of accurate determination. A microm- 

 eter eye-piece, Zeiss no. 4 ocular and objective D were em- 

 ployed. The measurements in this paper are in these microme- 

 ter units, each one of which is equivalent to 0.036 mm. Four 

 measurements were made upon each pluteus; the first, from the 

 posterior end of the right body-rod to the end of the right 

 anal arm (fig. a, A. R.); the second, from the end of the left 

 body-rod to the end of the left anal arm; the third, from the 

 posterior end of the right body-rod to the end of the right oral 

 arm (text fig. a, 0. R.) ; and the fourth, from the end of the 

 left body-rod to the end of the corresponding oral arm. Each 

 measurement thus included an arm and a body length, ex- 

 cepting in the young forms where the skeletons were still rep- 

 resented by the tri-radiate spicules. In this case the lina a-a' 

 (fig. b) would correspond to the first two measurements given 



