66 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



stages thai the spindle lies in it also in an anteroposterior direction. All of these 

 divisions of the endoderm are equal and non-differential-. 



Finally the median mesenchyme cells, B 75 . 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 7 - 6 ) 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 



Ectoderm 8th gen., 52 cells. 



Neural plate 8th " 12 " 



Dorsal hemisphere 



Endoderm 8th " 20 " 



Chorda 8th " 8 " 



Neural plate 8th " 8 " 



Muscle .... 9th gen., 8 cells. . . 8th " 4 " 



Mesenchyme. 9th " 4 " ...8th " 14 " 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. 136-143). The cells which divide first in 

 this period are the four median neural plate cells A 87 , A 88 (fig. 136) ; shortly after- 

 ward the four lateral ones A 815 , A 816 (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, 143), 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 often 

 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 side belong to the posterior quadrants (b 8-17 , b 818 , b 819 ', 

 b 8?0 ) ; the other six pairs (a 8-25 , a 826 , a 8 - 17 , a 818 , a 8-19 , a s ' 20 ) which form the median 



