ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 67 
anterior portions of these rows, are derived from the anterior quadrants, and they 
form the anterior portion of the neural plate. 
Finally in the stage shown in figures 144 to 147, these twenty cells divide, all 
the spindles being approximately dorso-ventral in direction. About the same time 
the eight chorda cells divide, the spindles standing in a dorso-ventral direction (fig. 
145), and a little later the six most anterior endoderm cells (А*?, A*?, АЧ) divide, 
the spindles being nearly transverse (fig. 147). "These thirty-four divisions advance 
the embryo from 184 to 218 cells. 
Beyond this point I have not attempted to follow the lineage of each and 
every cell; this could be done successfully only by a most exacting study of serial 
sections in connection with whole preparations. With sufficient labor and material 
I believe that the lineage of every cell could be traced through to the tadpole stage, 
but I have lacked both the time and the material for such a study. Tabulating 
the cells of this stage we find that there аге: 
Ventral hemisphere 
Ectoderm . . .9th gen., 104 узы 
Neural pinta un "VoM 
Dorsal hemisphere 
Endoderm.. .9th * 12 *....8th gen., 14 cells. 
Chorda- 04 ӨНҮ тщр и 
Neural plate . 9th “ 16 “ 
Мовне: = FARA қ. ^ СИС ШЫТ ER 
Mesaishy iid 9th * на, г CON pon Tth gen., 2 cells. 
9th gen., 184 сей». Sth gen., 32 cells. 7th gen., 2 cells. 
218 cells. 
At this stage the gastrula is nearly circular in outline when viewed from the 
dorsal or Ventral pole; the gastrocoel is a cavity deep and wide in front and nar- 
row behind, where it opens to the exterior through a deep groove between the 
mesenchyme and muscle cells of each side. The endoderm lies at a deep level, in 
contact with the ectoderm of the ventral side; in the anterior quadrants it consists 
of nine pairs of cells which become the gastric endoderm of the larva, in the pos- 
terior quadrants are four pairs of endoderm cells which meet along the median 
plane; the two posterior pairs become the caudal endoderm of the tadpole, the two 
anterior pairs are added to the gastric endoderm. Three pairs of cells at the hinder 
end of this cord of caudal endoderm are mesenchyme, while on each side of these 
and running forward lateral to the caudal endoderm are seven pairs of large mesen- 
chyme cells (in fig. 147 only five pairs are visible, since two lie at a deeper level, 
one beneath the cell B* and another beneath A*”). Тһе muscle rudiment consists 
of six pairs of large cells, dorsal to the mesenchyme, on each side of the blastopore 
groove and still uncovered by ectoderm. The chorda consists of sixteen cells arranged 
in two rows of eight each, one ventral to the other, and covered superficially by 
cells of the neural plate. Sixteen cells of the neural plate which cover the chorda 
