584 CELL DIVISION IN EGGS OF CREPIDULA. 



PLATE XLV. 



Cleavage of Isolated Blastomebes. 



Fig. 30. Exp. 906: Third cleavage in § or f blastomeres, separated by pressure; the cells are 

 smaller than the normal macromeres, owing probably to loss of yolk during pressure; each cell is dividing 

 dexiotropically as it should in the third cleavage, but the relative positions of spindles and cytoplasmic 

 areas in the two cells have undergone certain changes as may be seen by comparison with fig. 20. 



Fig. 31. Exp. 921: j or 1 blastomeres, separated by shaking; the spindles are normal in position 

 though division in B has been delayed. 



Fig. 32. Exp. 855: J blastomere, isolated by shaking; the first micromere has been formed and 

 the second is forming in typical manner. Original animal pole indicated by polar bodies. 



Fig. 33. Exp. 855: I blastomere, isolated by shaking; the first and second micromeres have formed 

 in normal manner, and the former is dividing as in a whole egg. Original animal pole indicated by polar 

 body. 



Fig. 34. No. 714: f blastomeres, probably separated by pressure; each macromere has given off a 

 micromere in dexiotropic direction, as in whole eggs. 



Fig. 35. Exp. 855: J or f blastomeres, separated by shaking; each has produced a first micromere 

 in dexiotropic and a second in Isotropic direction, and the former are dividing in lseotropic direction just 

 as in whole eggs. The micromere plate is a continuous one, without breaks. 



Fig. 36. No. 711: f blastomeres, one macromere having been separated, probably by pressure. 

 Each macromere has formed one micromere in normal fashion, but a gap exists between micromeres lc 

 and la. 



Fig. 37. Exp. 864: J or f blastomeres, separated by hypertonic sea water and then left in normal 

 sea water 10 hrs. Each macromere has formed three micromeres, and the first and second of these have 

 subdivided in normal fashion. The egg is a whole in the sense only that it shows no gaps where cells are 

 missing. 



Fig. 38. No. 715: \ egg, probably separated by pressure after third cleavage, as shown by the fact 

 that two micromeres of the first set (la and 16) are present. 



Fig. 39. Exp._ 875: f blastomeres, macromere C having been destroyed in dilute sea water; the 

 macromeres have given off the first micromeres, and these have subdivided in typical fashion, forming a 

 triangular, but continuous micromere plate. 



Fig. 40. _ Exp. 958: f blastomeres, separated by shaking. Cells A and D have given rise to first 

 and second micromeres in normal manner; B has formed only the first micromere and both IB and 16 lack 

 nuclei. 



Fig. 41. Exp. 867: f blastomeres, separated in hypertonic sea water. Typical cleavage of micro- 

 meres and macromeres of each quadrant represented, but the cells of one quadrant are wholly lacking. 



Fig. 42. Exp. 1002 (2) : f blastomeres, separated by pressure. Cleavage typical for each quadrant, 

 but delayed in quadrant B; the third micromere is just coming off from 2D. 



Fig. 43. Exp. 958: f blastomeres, separated by shaking. The cleavage of these three quadrants 

 A, C, and D is absolutely typical; the cells of the fourth quadrant (C) are entirely lacking, but there are no 

 gaps to mark the places from which they have dropped out. Each macromere has produced three micro- 

 meres, and in addition D has given rise to a fourth, the mesentoblast, 4d(= M 1 , M 2 ). The micromeres 

 have each divided in typical manner, giving rise to a cross (stippled cells) with three arms instead of four. 



Fig. 44. Exp. 959: § blastomeres separated by shaking; the cleavage of each macromere and the 

 subdivisions of each micromere have taken place as in normal eggs. 



PLATE XLVI. 

 Effects of Pbessuee. 



Fig. 45. Exp. 918: Egg pressed during formation of second polar body, which is abnormally large. 



Fig. 46. Exp. 918: Egg pressed during first cleavage which was rendered unequal; nuclei abnormally 

 lobulated. 



Fig. 47. Exp. 958: Egg shaken during first cleavage. Similar to preceding. 



Fig. 48. Exp. 901 : Very large yolk lobe at vegetal pole of one of the cells. 



Fig. 49. No. 724: Egg probably pressed during the first cleavage; second cleavage spindles abnormally 

 small. 



Fig. 50. Exp. 918: Pressed during second cleavage in the direction of the spindle axes; macromeres 

 A and B have divided normally giving rise to micromeres la and 16; in macromere CD the second cleavage 

 is still incomplete, and the entire amphiaster is abnormally large. 



Fig. 51. Exp. 911: Pressed during the first cleavage; three macromeres with multiple nuclei formed 

 probably as result of a triaster. 



Fig. 52. Exp. 911: Pressed during second and third cleavages in chief axis of egg; macromeres C and D 

 with their micromeres approximately normal; macromeres A and B have divided in abnormal planes 

 giving rise to macromeres 1A 1 and IB 1 instead of micromeres. 



Fig. 53. Exp. 911: Pressed during second and third cleavages in chief axis of egg; all the upper tier 

 of cells except lc are abnormally large and are dividing like micromeres of the first set; in IB 2 , 1C and ID 2 the 

 direction of division is like that in the formation of the 2d set of micromeres. 



