CELL DIVISION IN EGGS OF CREPIDULA. 505 



In general the method of procedure was to take eggs in a given stage of 

 development, subject them for a definite time to altered conditions, and then 

 return them to normal conditions where they were allowed to remain for varying 

 lengths of time. As just indicated most of the eggs were fixed and stained 

 according to the methods described in my previous papers on Crepidula (1897, 

 1902) . In practically all cases the eggs experimented on were, carefully compared 

 with others of the same laying kept under normal conditions. For descriptions 

 and figures of the stages in the normal development of Crepidula reference should 

 be made to the papers just mentioned. 



The present work will deal with the following topics in the order named: 

 (1) Abnormalities found in nature. (2) The cleavage of isolated blastomeres. 

 (3) The effects of pressure on nuclear and cell division. (4) Effects of electric 

 currents of varying strengths. (5) Effects of abnormal temperature. (6) 

 Effects of ether. (7) Effects of decreased oxygen tension in the medium. 

 (8) Effects of carbon dioxide. (9) Effects of hypotonic solutions. (10) Effects 

 of hypertonic solutions. 



I. Abnoemalities Found in Nature. 

 (Plates XLIII, XLIV. Preparations 942-945. 1 ) 



On the whole there are remarkably few cases of abnormal eggs in nature; 

 probably not more than one egg in a thousand shows any abnormality whatever. 

 In plates XLIII and XLIV are shown the principal types of such abnormalities. 

 Most of these, as for instance figs. 2-5, 9, 11-13, 15, and probably 18-26 are due 

 to pressure. Since the eggs are laid in very thin and delicate capsules and are 

 crowded close together, about 175 eggs being found in each capsule, it is really 

 surprising that a larger number of eggs do not show the effects of pressure. 



The most usual effect of pressure during mitosis is the formation of a lobe of 

 cytoplasm opposite one or both ends of the amphiaster (figs. 2, 12, 13, 20). Such 

 a lobe, as I have maintained in a previous paper (1902), is an evidence of 

 reduced tension in the cell membrane at the points opposite the ends of the 

 spindle. More recently, I (1912) have observed that lobes may be formed 

 opposite the poles of the future spindle, while the cell is still in a resting con- 

 dition; such a condition is shown in figs. 15 and 19, and indicates that the axis 

 of the future spindle is already marked out in the resting stage preceding divi- 

 sion, by points of reduced tension in the cell membrane. Whether this is the 

 chief or only cause of the orientation of the spindle in the cell, cannot at present 

 be affirmed or denied, but it seems very probable that it is at least one of the 

 factors determining such orientation. Associated with the formation of these 

 lobes the spindle itself is sometimes shifted toward one pole or the other so that 

 its equator no longer lies in the typical plane of cell division. In fig. 13 the mi- 

 totic figure has been moved so that one of the daughter nuclei lies in the division 



1 See Catalogue, p. 559. 



33 JOURN. ACAD. NAT. SCI. PHILA.. VOL. XV. 



