The Ascidian Half- Embryo. 53 



B. Cleavage of the ^ blastomere As is well known, the isola- 

 tion of an ascidian blastomere is effected by the death of its 

 neighboring cell or cells, and not by an actual separation. The 

 dead cell partially disintegrates and exerts upon the living cell no 

 modifying influence, such as mechanical obstruction to rounding 

 during division, etc. 



\ . At the normal time, viz : at the time of division of control 

 eggs, the injured blastomere divides about equally (figs. 4 and 13). 

 Often when the eggs are operated upon when passing into the 4- 

 cell stage, evidence of division in the dead cell will remain. In 

 such cases the division plane of the living cell is seen to be meri- 

 dional and at right angles to the first. Therefore, it corresponds 

 with the second cleavage-plane of the normal embryo. In all 

 cases where it is possible to ascertain the facts this relation ob- 

 tains. Driesch finds in Phallusia that no such constancy of rela- 

 tion exists. 



|-. After a normal period of rest the two cells divide at the 

 same time. There are thus produced four cells which are ar- 

 ranged in a manner exactly similar to the half of a normal 8- 

 celled embryo. Seen from the side (figs. 5, 0) the cells lie so that 

 two are separated, while two are in contact ; these latter are the 

 posterior dorsal and the anterior ventral cells, as shown by the 

 succession of the cleavage planes of the fragment. Precisely as 

 in the normal 8-celled embryo, there is an anterior shifting of 

 the dorsal cells upon the lower cells. According as this shifting 

 is to the right or left, in lateral view, one is confronted ~by a right 

 or left half-embryo. From a comparison of the figures, it is seen 

 that the embryo in fig. 5 is the same as the half turned toward the 

 observer of fig. 1 ; while that shown in fig. 9 is derived from a 

 right -^ blastomere. The appearance of the f embryo in end view 

 is shown in fig. 14, and a characteristic crossing of the spindle 

 axes is exhibited, which is similar to their crossing in the com- 

 plete egg (vide Castle for figures). The four-celled fragment, 

 then, is in nowise a counterpart of the normal four-celled embryo, 

 but, on the contrar3 r , corresponds in every particular to the half of 

 an eight-celled embryo. 



From Chabry's fig. 106, it appears that a typical -| stage occurs 

 also in Ascidiella. 



-j^-. At the next cleavage, all the cells divide (figs. 6, 10). 

 Exactly as in the origin of corresponding normal cells, (fig. 2) 



