66 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
stages that the spindle lies in it also in an antero-posterior direction. All of these 
divisions of the endoderm are equal and non-differential. 
Finally the median mesenchyme cells, В”, divide, the spindles being antero- 
posterior (fig. 136), and the resulting daughter cells alike in size and quality. 
With this division all the cells of the embryo have passed into the eighth 
generation, except the small posterior mesenchyme cells (B®) which never again 
divide, so far as I have observed; and excepting eight muscle cells and four mesen- 
chyme cells which have passed into the ninth generation. The following tabular 
statement summarizes the character and location of the cells at the close of this stage: 
Ventral hemisphere 
boe i = Le CaS A Sth gen., 52 cells. 
Neural plate... . . ..... Trace cL LOIS Pn 
Dorsal hemisphere 
Ehdodonü eoe o o VE Bb "0E Ө 
CUN LU E A A E 8th * 8e 
Neuial nina ылы us Bb. n S. 
Muscle ....9th gen., 8 cells... 8th ©“ с р 
Mesenchyme, Hh Ж ое р " M 7 7th gen., 2 cells. 
9th gen., 12 cells. — 8th gen., 118 cells. — 7th gen., 2 cells. 
132 cells. 
The 132-cell stage is not a sharply defined one, for before all the divisions 
which have been described above have been finished, other divisions are begun 
which lead to the 184-cell stage (figs. 156-145). Тһе cells which divide first in 
this period are the four median neural plate cells А7, A*S (fig. 136); shortly after- 
ward the four lateral ones A*™, А%9 (fig. 140), also divide. The spindles in all | 
these cells lie in a radiating position around the blastopore, and as a result of this 
division there are produced in the dorsal hemisphere two rows of neural plate cells, 
eight cells in a row, situated at the anterior border of the blastopore and dorsal to 
the chorda cells. 
About the same time forty-four of the fifty-two ectoderm cells divide; the 
spindles are approximately transverse in all these cells, except in the most posterior 
row of the ventral hemisphere, where they are dorso-ventral, and in two transverse 
rows of four cells each, which are the third and fourth rows in front of the animal 
pole (figs. 139, 145), where the spindles are antero-posterior in direction. 
By these divisions the dorsal neural plate cells are increased to sixteen, and 
the ectoderm to one hundred and eight cells, so that at this stage the entire embryo 
contains one hundred and eighty-four cells. Twenty cells, forming two rows of ten 
each around the anterior border of the embryo just ventral to the equator, remain 
undivided for some time and are conspicuous for the large size of their resting 
nuclei and their more deeply staining cytoplasm (figs. 138, 142). The four hind- 
most of these cells on each € belong to the posterior quadraftw(h*" b*!8, p*19. 
Ь°®); the other six pairs (a9, 3975, 349.7, 3*5, 3*7, 35?) which form the 1. dn 
