CELL DIVISION IN EGGS OF CREPIDULA. 585 



Fig. 54. Exp. 915: Pressed during 2d and 3d cleavages in chief axis of egg; macromere A divided 

 nearly equally into 1A 1 and L4 2 , which are now forming micromeres of the 1st set; macromeres IB, 1C 

 and ID are forming micromeres of the second set. 



Fig. 55. Exp. 904: Pressed during 3d cleavage, the direction of which was changed from dexiotropic 

 to Isotropic in all quadrants except B; correspondingly the next cleavage (shown by spindles) is dexiotropic, 

 instead of Isotropic as in typical eggs. 



Fig. 56. Exp. 915: Pressed in chief axis during the 4th cleavage, the chief result being the enlarged 

 size of 2a and 2c. 



PLATE XLVII. 

 Effects of Pressure. 



In all figures except 57, 65, 66 the axis of pressure was in the direction of the egg axis. 



Fig. 57. Exp. 1003: Pressure parallel with the egg axis has produced a linear arrangement of the 

 four macromeres, each of which preserves its original polarity and is dividing to form the first set of micro- 

 meres. 



Fig. 58. Exp. 1003 : Pressure in the chief axis of the egg has led to the formation of the micromeres 

 of the 1st and 2d sets between the macromeres, instead of above them. The micromeres are larger than 

 usual and the 1st set has subdivided giving off "turret" cells two of which (1& 2 and Id 2 ) are much larger 

 than usual, while the other two (la 2 and lc 2 ) have been forced to the lower side of the egg. 



Fig. 59. Exp. 915: Pressure in the direction of the egg axis has led to the formation of eight macro- 

 meres, each of which is giving off in a dexiotropic direction a micromere of the 1st set. 



Fig. 60. Exp. 915: Pressure in the egg axis during the 4th cleavage has caused the formation of 

 larger micromeres than normal, especially in quadrants C and D, indeed the 2d division of C is nearly equal, 

 giving rise to two macromeres; in the subdivisions of the 1st set of micromeres the "turret" cells (la^-ld 2 ) 

 are much larger than usual. 



Fig. 61. Exp. 915: In this case the pressure was probably applied after the formation of the 1st set 

 of micromeres which are normal; the 2d set is also normal except in quadrant A, where the macromere \A 

 divided nearly equally into macromeres 2A 1 and 2A 2 , and the former has divided into 2A 1 and 2a 1 . 



Fig. 62. Exp. 1001 (1): Normal except that macromere C divided equally at its first division; the 

 right upper half then gave off a micromere of the first set (lc 1 ) which, judging by the shape of the cell, is 

 about to form a "turret" cell as in the other three quadrants. 



Fig. 63. Exp. 1004 (2) : Pressed during the 3d cleavage, macromeres A and D divided nearly equally, 

 thus increasing the number of macromeres to six, each of which has formed a micromere of the 1st set, while 

 C has produced also a micromere of the 2d set (2c). 



Fig. 64. Exp. 1003: Pressed during the formation of the 2d set of micromeres, which are much larger 

 than usual; in the subdivision of 16 and lc the peripheral products ("turret" cells) have been forced to 

 the lower pole of the egg, and the macromeres have been pushed apart as in fig. 58. 



Fig. 65. Exp. 915: Compressed obliquely to the egg axis; macromere B being shoved under the 

 micromere plate; macromere A formed a first micromere larger than normal, which has divided equally 

 (la 1 , la 2 ), and then gave rise to a second "micromere," which is really a macromere (2A 1 ). 



Fig. 66. Exp. 1001 (1): Compressed parallel with the egg axis, B and D being shoved under the 

 other cells; the 1st and 2d sets of micromeres are nearly normal; at its 2d division A divided nearly equally 

 into 3A and 3A 1 , each of which has formed a micromere (2a, 2a) in a dexiotropic direction. 



Fig. 67. Exp. 1001 (3): The cleavage is normal except that 4d is larger than usual, the result of 

 pressure in the direction of the egg axis; the cells of the ectodermal cross are stippled. 



PLATE XLVIII. 

 Effects of Electric Current. 



Eggs subjected to electric current (except figs. 76, 77) but probably showing effects of pressure. 



Fig. 68. Exp. 1104 (?): 5 mil. amp., 5 min. First maturation spindle; the mitotic figure is much 

 longer than normal, is central in the egg and the spindle fibres, centrosomes and astral rays are either lacking 

 or very faint. The position is probably due to pressure. 



Fig. 69. Exp. 1121 (2): 5 mil. amp. 10 min., normal 3 J hrs. Second cleavage spindles abnormal 

 in position, first cleavage furrow incomplete, development stopped. 



Fig. 70. Exp. 1121 (2): Similar to preceding. 



Fig. 71. Exp. 1121 (2): Similar to preceding. 



Fig. 72. Exp. 1121 (2): Similar to preceding. 



Fig. 73. Exp. 1121 (2): Two cells, one containing a complete amphiaster but without any chromatin, 

 the other containing two nuclei and one sphere, probably as the result of pressure at the close of the 1st 

 cleavage. 



Fig. 74. Exp. 1121 (2): Second cleavage spindle abnormal in position and division delayed in one 

 blastomere. 



Fig. 75. Exp. 1121 (2): Three macromeres in a linear series, the middle one containing two separate 

 cytoplasmic areas, nuclei and spheres, the result of the suppression of the first cleavage, as in fig. 72. 



Fig. 76. Exp. 1001 (3): A pressure experiment included in this plate by mistake, showing eight 

 macromeres and eight micromeres. 



