532 CELL DIVISION IN EGGS OF CREPIDULA. 



described and are more difficult of interpretation. In fig. 124, three macromeres 

 were formed in a row, the one farthest to the right being approximately normal; 

 it contains one nucleus and sphere and has given off two micromeres, the first of 

 which was formed lseo tropically and is now dividing, and the second is just coming 

 off dexiotropically. The middle macromere contained a polyaster and gave off 

 a large micromere with three nuclei, adjoining the polar bodies, and it now 

 shows a polyaster in its second cleavage (fourth general cleavage). The macro- 

 mere to the left contains a perfect achromatic spindle but no trace of chromatin; 

 evidently a centrosome without a nucleus went into this macromere at the 

 second cleavage, while the middle macromere received two or more nuclei and 

 centrosomes. 



A somewhat similar case is found in fig. 126; there are here three macromeres, 

 one of which (right) contains an amphiaster and a triaster and has just given off 

 two micromeres of the second set on opposite sides of the large micromere of the 

 first set which was formed at the previous cleavage; another macromere (left) 

 has just given off in dexiotropic direction a micromere of the second set, while 

 the micromere of the first set which was formed from this macromere at the previ- 

 ous cleavage is now dividing and contains two centrosome and two equatorial 

 plates; the third macromere (above) contains several centrosomes and spheres 

 but no trace of nuclear material. 



Figs. 127 and 130 show two very irregular cleavage forms, in which the cell 

 membranes bulge out irregularly in lobes or threads; even the polar bodies in fig. 

 127 are connected with the contiguous cells by such threads. The very irregular 

 cleavage shown in these two figures is probably due to these abnormal surface 

 tension phenomena, which lead to the formation of cells which are atypical in 

 size and position. 



The figures shown on Plate LII are but a few of the almost infinite variety of 

 abnormalities which resulted from the experiments with carbonic acid. It seems 

 to me that these results indicate the importance of surface tension phenomena in 

 mitosis and development, the importance of cell walls in preventing the interference of 

 one mitotic system with another, and the connection between localized reductions in 

 surface tension and the direction and position of mitotic spindles. 



IX. Effects of Diluted Sea Water. 



(Plates LIII, LIV. Exps. 858, 859, 870-876, 954-956, 993, 1182-1186.) 



In testing the action of various changes in the environment on nuclear and 

 cell division in Crepidula, a number of different experiments were made on the 

 effects of diluted sea water. In the case of animals living in shallow water along 

 shore, as Crepidula and its messmate Pagurus do, it must frequently happen 

 that the density of the sea water is much reduced by heavy rains or by streams of 

 fresh water. Do these eggs show an adaptation to such conditions? I found that 

 a dilution of x /i or J^ part fresh water to 1 part sea water produced little if any 



