ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 03 
direction of the spindles in these cells is indicated by the equatorial plates repre- 
sented in figures 198 to 203. Іп the posterior quadrants all the spindles are 
nearly antero-posterior in direction, except in those three ventral cells on each side 
(b**, b*'9, ЪЁ!) which lie nearest to the dorsal surface and between the muscle cells 
behind and the neural plate cells in front (figs. 201, 203); in these cells the spin- 
dles are nearly vertical. Іп the anterior quadrants the spindles are antero-pos- 
terior in a transverse row of four cells which lies just in front of the animal pole 
(a, 2575): in the row of four cell just in front of this the spindles are transverse 
(а, а“); they are also transverse in a single pair of cells which meet at the mid- 
line just in front of the last mentioned row (a*!); in the most anterior row of the 
ventral hemisphere, consisting of six neural plate cells (а?! а”, a’), the spindles 
are dorso-ventral in position, therefore, at the close of this division there are 
twelve neural plate cells in the ventral hemisphere, arranged in two rows of 
six cells each. 
АП of these divisions of the cells of the ventral hemisphere are synchronous, 
equal and non-differential, and they increase the number of cells in the ventral 
hemisphere to sixty-four and bring the whole number of cells in the embryo up to 
one hundred and eight. 
Very soon after these divisions in the ventral hemisphere the posterior muscle 
cell (B^?) and one of the mesenchyme cells (В?) of each side divide; in the former 
the spindles converge posteriorly and ventrally toward the median plane, in the 
latter posteriorly and dorsally. Ву this division two mesenchyme and two muscle 
cells are added to the total in the embryo which at this stage consists of one hun- 
dred and twelve cells (fig. 133, 134") which may be tabulated as follows : 
Ventral hemisphere 
Ectoderm . . 8th generation, 52 cells. 
Neural plate 8th generation, 12 cells. 
Dorsal hemisphere 
MENU Dep LLLA uou E Tth gen., 10 cells. 
Chorda . . . . 8th generation, 8 cells. 
Neural plate . 8th е ale 
Muscle ... . 8th Bu 
Mesenchyme 8th » B АНГ)". tr b 
Sth generation, 96 cells. 7th gen., 16 cells. 
112 cells. 
The only cells in the entire embryo which have not passed into the eighth 
generation at this stage are the ten endoderm cells, and six mesenchyme cells, two 
of which are anterior, and four median and posterior; all of these cells except two, 
the small posterior mesenchyme cells (В?®), divide soon after this stage and thus 
pass into the eighth generation, but not until after other cells have passed into 
the ninth. This stage, therefore, may be taken as representing, as nearly as may 
be found, the close of the seventh cleavage and the transition to the eighth. 
` ! The division in a pair of mesenchymé cells (B77) of this stage is not completed ; therefore, in 
the explanation of figures, this is called a 110-cell stage. 
