Mosaic Development in Ascidian Eggs. 157 



I. Right or Left Half Embryos (Figs. 13-33, 3^~4^)- 

 a. Cleavage. 



When the right or left half of an egg is injured in the 2, 4 or 

 8-cell stage, the other halt continues to segment in a normal 

 manner, provided it was not also injured. I have traced the cell- 

 lineage of these right or left half embryos up to the eighth genera- 

 tion of cleavage cells (the 112-cell stage of normal, eggs), while I 

 have determined the lineage of many individual cells as late as the 

 ninth or tenth generation (218-360 cell-stage). The cell-lineage 

 of these half embryos is essentially like the right or left half of a 

 normal egg, except that the direction of division and consequently 

 the position and size of some of the blastomeres may be slightly 

 altered. 



This alteration in the direction of cleavage is most evident in 

 cases where the egg was injured in the 2-cell stage, and it is prob- 

 ably due to the fact that the uninjured blastomere in such cases 

 becomes nearly spherical in shape, and does not remain hemi- 

 spherical as in the normal egg. Owing to this fact the median pole 

 of certain cleavage spindles, i. e., the one next to the original 

 median plane, is shifted toward the middle of that plane. The 

 resulting mass of cells is, therefore, more nearly spherical than in 

 the half of a normal embryo. (Figs. 13-20.) If the injury occurs 

 in the 4-cell stage or later, the change in the direction of the early 

 cleavages is not so evident as when it takes place in the 2-cell 

 stage. In case .one of the blastomeres was injured at the close of 

 the first cleavage, the direction of the karyokinetic spindles of 

 the second and third cleavages are entirely normal, since in both 

 these cases they lie parallel with the first cleavage plane, Fig. 

 13; but in the fourth cleavage in which one pole of the spindles 

 lies nearer that plane than the other, the median pole is shifted 

 toward the middle of that plane and consequently the cells 

 formed along the median plane come into closer contact with 

 one another and the cell aggregate is more nearly spherical 

 than in the right or left half of a normal i6-cell stage. (Figs. 

 14, 15, 21, 22.) These results entirely agree with those of Chabry 

 and Crampton. 



The fifth cleavage of the right or left half embryo is also like 

 the normal except in the direction of a few of the divisions; e. g., 

 Fig. 16 is nearlv normal but in Fig. 17 the division of the cell 



