CELL DIVISION IN EGGS OF CREPIDULA. 527 



for 6 hrs., the results are the same as in the preceding experiment, except that 

 large sphere granules are present and are scattered through the cell and surround 

 the nuclei. Chromatin is clumped within the resting nuclei; chromosomes are 

 also clumped in the spindles. Centrosomes, spindle fibers and astral rays may 

 be present, but mitosis is very irregular, poly asters often being present; in other 

 eggs spindle fibers and astral rays disappear. These results are, in general, simi- 

 lar to those obtained by Gerassimoff (1892), on Spirogyra, and by Hacker (1900) 

 and Schiller (1909) on the segmenting eggs of Cyclops; in all of these cases the 

 spindle fibers were caused to disappear by the use of ether, but whereas these 

 authors observed that the division continued by a kind of amitosis after the dis- 

 appearance of the spindle, I have seen no evidence of real amitotic division in 

 Crepidula. Irregularities in the movements of the chromosomes with the con- 

 sequent scattering of chromosomes along the entire length of the mitotic figure 

 does sometimes lead to a chromatic connection between daughter nuclei. Such 

 modified mitoses, which are suggestive of amitosis, are found in many of these 

 experiments and are described more fully on pages 535, 548, 550 and 557. 



VII. Effects of Decreased Oxygen Tension. 



(Plate LI.) 



In view of the fact that the centrosomes and the surrounding spheres move 

 regularly during telokinesis to a free surface of the cell where the sphere sub- 

 stance spreads out under the cell membrane, it seemed desirable to determine 

 whether this movement is due to oxidations at the surface of the cell and conse- 

 quently to the presence of free oxygen in the surrounding water. Accordingly, 

 the oxygen tension of the water was reduced in two ways, first by boiling the 

 water in flasks or test tubes to drive off contained gases, after which the tubes 

 were stoppered and cooled before the eggs were placed in them, second by running 

 a current of purified hydrogen gas through a series of wash bottles in which eggs 

 were placed. 



1. Boiled Sea Water. 



(Figs. 106-109, 112-114, 116. Exps. 935-940, 947-953, 1010-1022.) 



Pure sea water was boiled vigorously for a few minutes in order to drive off 

 contained gases, but not so much as to lead to any appreciable concentration of 

 the salts. Flasks or test tubes of this water were carefully closed with rubber 

 stoppers which dipped into the water and consequently left no air space in the 

 tubes. The water was then cooled to the temperature of the water in the aquaria 

 and the eggs were gently introduced into the tubes. In thirteen experiments of 

 this kind (Nos. 935-940, 947-953) the tubes were left unstoppered and eggs in 

 stages varying from 1 cell to 24 cells were put into the tubes and left in them for 

 periods varying from 4 to 48 hours. Eggs which had been 48 hours in the boiled 

 water were generally becoming abnormal and some of them were dead, but all 

 eggs which were in the tubes from 12 to 24 hours were normal in form, or nearly 



