CELL DIVISION IN EGGS OF CREPIDTJLA. 529 



much larger than normal, and the spheres and centrosomes consist of large, 

 scattered granules; if the development has been halted during mitosis, the 

 spindles, centrosomes, and chromosomes remain distinct but the polar rays 

 disappear, being drawn into the central portion of the amphiaster (figs. 110, 111, 

 115, 117). If development has been stopped for many hours (18-48 hours), 

 cytoplasm may move toward the vegetal pole along the first and second cleavage 

 planes (fig. 116), while in other cases yolk may appear around the centrosomes 

 nearest the animal pole (fig. 118). 



Finally, by the aid of Dr. A. P. Mathews, I was able to employ in certain 

 experiments (Nos. 1027-1029), atmospheres composed of known quantities of 

 air and hydrogen. In one case (No. 1027), the proportions were 60 vols, of 

 hydrogen to 40 vols, of air; in two others (No. 1028, 1029), the proportions were 

 70 vols, of hydrogen to 30 vols, of air. In all of these experiments development 

 was stopped within a few cleavages, though the eggs remained in the bottles from 

 12 to 24 hours. 



To sum up, it may be said that the first effect of reduced oxygen tension is 

 the slowing of development and of cell division, and as a result of this retardation, 

 the resting nuclei, which continue to imbibe materials from the cytoplasm, 

 become very large and achromatic; at the same time, the nucleoli grow larger 

 than normal, as is always the case when the resting period is prolonged. The 

 nuclei and nucleoli thus come to resemble the germinal vesicle and germinal spot 

 of immature egg cells, and I believe that this resemblance is a real, and not merely 

 an apparent one. The spheres of resting stages also lose their definite outlines 

 and are resolved into large granules which are more or less scattered. In division 

 stages, mitotic figures may persist for many hours (18-48 hrs.) ; the spindle fibers 

 remain distinct and the spindle may shrink and at the same time stain more 

 deeply than normally; the polar radiations disappear and the centrosomes 

 frequently enlarge. Later stages of cleavage are not as much affected as are 

 earlier stages. 



In all cases where the experiment has not been prolonged more than 24 hours, 

 when eggs are returned to normal sea water development is resumed and the 

 nuclei and cells become normal in appearance. In some cases the micromeres 

 cease to form a one-layered epithelium, and become individually rounded; 

 collectively, they form a mass more than one cell thick, as I reported in a former 

 paper (1902, p. 102). Later work has shown that this result is exceptional rather 

 than usual, and that it occurs in many other experiments in which eggs are 

 degenerating (framboisia of Roux). 



I conclude therefore that the presence of oxygen in the medium is not the 

 chief cause of the movements of telokinesis. 



35 JOURN. ACAD. NAT. SCI. PHILA, VOL. XV. 



