CASTLE : EMBRYOLOGY OF CIONA INTESTINALIS. 259 



next two anterior sections. Of the eight cells mentioned, A^-^^, A^-^*, 

 J^'^^, and 5^-^* (cf. Fig. 82) are all in mitosis, but the four more laterally 

 situated and smaller ones are still quiescent. 



Considering as a whole the mesenchyme of this embryo, we see that it 

 consists of two lateral bands which have elongated with the elongation 

 of the embryo. They now extend through ten different sections from 

 near the anterior end of the embryo to a region posterior to the blasto- 

 pore (Fig. 89). The muscle cells, on the other hand, are gathered into 

 a pretty compact mass at the sides of and posterior to the blastopore 

 (Figs. 88-90). 



In the subsequent stages of development the portion of the embryo 

 lateral and posterior to the blastopore will be rapidly drawn out to 

 form the tail of the larva, while the portion anterior to the blastopore 

 will form the trunk. This will not come about, however, without a con- 

 siderable shifting of cells from one portion into the other, for the chorda 

 cells, which now lie anterior or lateral to the blastopore, must in large 

 part pass into the tail, while the mesenchyme cells, which are more ven- 

 trally located, and some of which now extend behind the blastopore, 

 will all pass forward into the trunk region. 



An examination of Figure 98 (Plate XII.) may help to give a clearer 

 idea of the stage just described. This figure shows a section made 

 nearly parallel to the sagittal plane, but a little to one side of it, through 

 an embryo slightly older than the one last under discussion (Figs. 88- 

 92). The anterior chorda fundament, it is seen, has been carried back 

 beyond the middle of the . embryo's dorsal surface. The muscle cells 

 have been forced backward and downward into a nearly vertical po- 

 sition behind the blastopore, and are nearly covered over with ectoderm 

 (cf. Fig. 93). 



Numerous cell divisions have recently occurred in the ectoderm, and 

 the number of endoderm cells has also plainly increased. A very marked 

 elongation of the embryo has attended these divisions. Several cells in 

 the medullary plate are also dividing. On account of the slight obliquity 

 of the plane of sectioning, the small posterior cells C-^, IP-^ (ms'chy.), 

 do not actually appear in this section as represented, but have been pro- 

 jected there from the adjacent section. In that section the endoderm 

 extends back in a double row of cells into contact with C'^, D''-^, as at 

 the stages shown in Plate XL Figs. 78 and 79. 



In Figures 93-97 (Plate XII.) are represented five cross sections 

 through an embryo in about the same stage as is shown in Figure 98. 

 The approximate position of the sections in the embryo is indicated on 



