CELL DIVISION IN EGGS OF CREPIDULA. 511 



meres from the animal pole differs from that of blastomeres from the vegetal 

 pole. On the other hand practically all investigators agree that the nuclei of 

 the cleavage cells are not only totipotent but that they are qualitatively all 

 alike. The following experiments on the effects of pressure on Crepidula eggs 

 confirm in the main the conclusions reached by previous investigators. 



1. Modifications of the Cleavage Pattern. 



The eggs of Crepidula plana are laid in thin, transparent, membranous 

 capsules; about 175 eggs are found in each capsule, and the walls of the capsules 

 are usually flattened together in one plane so that opposite walls are in contact 

 at the middle of the capsule while the eggs are crowded into the ring-shaped 

 space at its periphery. Owing to the crowding together of the eggs in these 

 flattened capsules it sometimes happens that eggs which have not been subjected 

 to experiment show the effects of pressure, as previously mentioned, but such 

 modifications are relatively rare in a state of nature. In most of the experiments 

 the eggs were subjected to pressure within the capsules; in some instances 

 (Exps. 897, 900-908, 910, 911, 916-920) they were removed from the capsules 

 before being subjected to pressure. Whether the eggs were pressed within the 

 capsules or after removal from them the results were essentially the same. In 

 general eggs which had been removed from the capsules were more frequently 

 broken and injured than when they were pressed within the capsules. The 

 capsules, or freed eggs, were placed under cover glasses, or between glass slides 

 in dishes of fresh sea water, or they were placed in a Ziegler compressorium 

 through which a current of water was kept running, and were allowed to remain 

 in such positions for from four to eighteen hours. After removal of pressure the 

 eggs were either fixed at once or were allowed to develop under normal conditions 

 for periods of time varying from one to sixteen hours. Varying degrees of 

 pressure were thus applied in different axes and for various lengths of time to 

 eggs in different stages of development. In most cases pressure was applied 

 until the spherical eggs became flattened disks, and it was surprising to note 

 how much flattening the eggs would bear without bursting. 



In cases where pressure was applied during the maturation of the egg, and 

 parallel with its chief axis, the polar spindle may remain near the center of the 

 egg (fig. 68) and the polar bodies may be abnormally large, as in fig. 45 where the 

 second polar body is as large as an ordinary micromere; or a large "yolk lobe" 

 may be formed at the vegetal pole. In cases where large polar bodies are formed 

 their nuclei are usually correspondingly large. Not infrequently all division is 

 stopped during pressure and I think that in such cases the pressure is most 

 frequently in the direction of the normal spindle axis. 



The results of pressure during the cleavage of the egg are essentially the 

 same, so far as cell division is concerned, as during the maturation, i. e., the 

 spindles may be turned out of their normal positions, the relative sizes and 

 positions of the daughter cells may be altered, the cleavage may be suppressed, 



