60 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



civd by their sister cells (B 75 , B 75 ). Thus the vegetal pole is slightly posterior to 

 the middle of the dorsal lace and the animal pole is decidedly posterior to the mid- 

 dle of the ventral face in all of the stages mentioned, and this condition becomes 

 even more pronounced in later stages; thus in the 124-cell stage (fig. 139) there are, 

 ventral to the equator, six pairs of cells adjoining the mid-line in front of the animal 

 pole and four behind it. while in the 184-cell stage (fig. 143) there are eight pairs 

 of such cells in front of the animal pole and five behind it; in the 2lN-cell stage (fig. 

 144-147) there are ten such pairs of cells in front of the animal pole and only five 

 behind it. All of the cells of the ventral hemisphere are of approximately the same 

 size, so that in these later stages it is evident that the animal pole lies far back of the 

 middle of the ventral hemisphere. This location of the animal pole posterior to the 

 middle of the ventral hemisphere is due in the first instance to the smaller size of 

 the posterior cells in the 4-cell stage and then to the fact that the prevailing position 

 of the spindles in the anterior cells of tins hemisphere is parallel with the median 

 plane, while in the posterior cells it is transverse. It is not due. as might at first 

 thought seem to be the case, to the more rapid growth and division of the anterior 

 cells of the ventral hemisphere since all of these cells divide at nearly the same 

 time and are of approximately the same size. The irrespective significance of this 

 eccentric location of the animal pole maybe found in the greater length of the ante- 

 rior lip of the blastopore, as compared with the posterior lip. 



C. Gastrulation ; Seventh to Ninth Generation of Cells, 64-218 Cells. 

 In both Ciona and Cynthia the gastrulation actually begins during the seventh 

 cleavage and it is far advanced by the close of the eighth, though the closure of the 

 blastopore and the completion of the gastrulation does not occur until about the end 

 of the tenth cleavage. 1 have followed the lineage of every cell through the seventh 

 cleavage and of almost all the cells through the eighth, and have therefore been able 

 to determine the part which each cell takes in the formation of the gastrula. At 

 no time after the 64-cell stage are all the cells of the embryo in the same generation. 

 From this time toward the endoderm cells lag behind the ectoderm and mesoderm 

 cells in division; the eighth cleavage occurs in the ectoderm and mesoderm before 

 the seventh is finished in the endoderm. Therefore the periods of the seventh and 

 eighth cleavages cannot be sharply separated, but for the sake of convenience we 

 shall consider these two cleavages as if they were distinct. 



7. Seventh Cleavage ; 64-76, 76-112 cells. (Figs. 46-51, 130-139. 198-204.) 

 The seventh cleavage begins in the anterior quadrants of the dorsal hemisphere 

 in the two pairs of chorda cells (A 7:i , A 77 ) and in the two pairs of neural plate cells 

 (A 74 , A 78 ); in the posterior quadrants it begins in the two most anterior cells of the 

 m\scent on each side, the pair of muscle cells, B 74 . and the pair of mesenchyme 

 cells, B 73 (figs. 130-132). With the exception of the two mesenchyme cells the 

 spindles in all of these cases are parallel with the plane of the equator and with the 

 surface of the egg; in the mesenchyme cells the spindles, when seen from the dor- 



