60 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
ered by their sister cells (B75, B®). Thus the vegetal pole is slightly posterior to 
the middle of the dorsal face 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 218-cell stage (fig. 
144-147) there are ten such pairs of cells in front of the animal pole and only five 
behind it. АП 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 this 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. Тһе prospective significance of this 
eccentric location of the animal pole may be 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 ОҒ CELLS, 64-218 CELLS. 
In both Crona and Cynthza 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. I 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 foward 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. 
T. 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*?, A**) and in the two pairs of neural plate cells 
(А?*, A75) : in the posterior quadrants it begins in the two most anterior cells of the 
crescent on each side, the pair of muscle cells, B^*, and the pair of mesenchyme 
cells, D? (figs. 150—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- 
