ORGANIZATION AM) CELL-LINEAGE OE 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. 

 1 Km. and a little later the six most anterior endoderm cells (A s -, A 83 , A 8,4 ) divide, 

 the spindles being nearly transverse (fig. 147). These thirty -four divisions advance 

 the embryo from 1S4 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 are : 

 Ventral hemisphere 



Ectoderm . . . 9th gen., 104 cells. 



Neural plate . 9th " 24 " 

 Dorsal hemisphere 



Endoderm. . . 9th " 12 " . . . . 8th gen., 14 cells. 



Chorda 9th " 16 " 



Neural plate . 9th " 16 " 



Muscle 9th " 8 " .... 8th " 4 " 



Mesenchyme . 9th " 4 " . . . . 8th " 14 " 7th gen., 2 cells. 



9th gen., 184 cells. 8th 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 814 and another beneath A 8 - 12 ). The muscle rudiment consists 

 of six pairs of large cells, dorsal to the mesenchyme, on each side of the blastopore 

 en >ove and still uncovered bv 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 



