70 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



the eighth cleavage there is therefore a regular alternation in the direction of 

 division in most of the cells. 1 have not observed the tenth cleavage of these 

 cells, but it seems probable that the direction of division is, in many of the cells, 

 the same as at the ninth cleavage, if one may judge by the longitudinal rows 

 of cells as well as by the number of rows which are present in the posterior 

 quadrants (figs. 155, 160). The animal pole is, therefore, shifted still further 

 forward during this cleavage. The posterolateral ectoderm cells slowly overgrow 

 the muscle cells, but for a long time they do not overgrow the median posterior 

 mesenchyme cells, and there is therefore left the deep notch in the blastopore at 

 the hinder end of the embryo which has already been described. 



The neural plate in the stage shown in figure 148 consists of six transverse 

 rows of cells, only four of which show in the figure. The two posterior rows are 

 derived from the dorsal hemisphere and consists of eight cells each; the four ante- 

 rior rows consist of six cells each, anil are derived from the ventral hemisphere. 

 In subsequent divisions of the posterior rows of this plate the spindles are antero- 

 posterior in direction, thus adding to the number of rows but not to the number of 

 cells in each row; for example, in figure 152 there are eight rows of cells, but 

 apparently only six cells in each row. 1 



The endoderm cells of the anterior quadrants divide chiefly in a transverse 

 plane; those of the posterior quadrants in an antero-posterior plane (figs, 150, 

 151, 156). This fact, taken in conjunction with the direction of division in the 

 ectoderm cells, contributes to the lengthening of the posterior part of the embryo 

 and to the widening of the anterior part, and consequently to the shifting of the 

 animal pole further toward the anterior end. 



Of the two rows of chorda cells established at the eighth cleavage one has 

 come to lie posterior to the other, and both bend so as to become horse-shoe-shaped 

 (fig. 153). Later these cells divide again (fig 157) and, pushing backward with the 

 anterior lip. carry the muscle cells before them, as already described. 



In the 218-cell stage there were twenty mesenchyme cells: in the next stage 

 shown (fig. 150) these are increased by one or two divisions so that there are twenty- 

 two or twenty-four cells. As in the preceding stage, they still lie on the ventral 

 side next to the ectoderm and along the posterior border of the gastral endoderm. 

 In figure 150 only one row of mesenchyme cells is found lateral to the caudal endo- 

 derm ; in figures 154 and 15G there are two such rows. In all these figures there 

 are three pairs of mesenchyme cells at the hinder end of the caudal endoderm ; the 

 most posterior of these is the small jxtsterior mesenchyme cells (B 76 ), the others are 

 B 89 and B 810 . All of these cells are protoplasmic, stain deeply and art' strikingly 

 different in appearance from the endoderm cells. 



The muscle cells, which in the 218-cell stage consisted of six pairs of cells, are 

 shown in figures 51 and 153 increased to eight pairs which are arranged in two 

 rows on each side of the blastopore groove. By the continued growth of the ante- 



1 In this figure it is possible that a single row of cells on each side of t lie .stippled area should be 

 reckoned as part of the neural plate. 



