castle: embeyology of ciona intestinalis. 255 



pore, but they are much larger than the nerve cells. They now lie lateral 

 and posterior to the blastopore. (See Figs. 81-83 ; cf. Figs. 79 and 80.) 



The backward growth of the anterior lip of the blastopore has carried 

 the crescent-shaped anterior chorda fundament (Fig. 71) from its origi- 

 nal position to about the middle of the embryo's dorsal surface (Fig. 81). 

 It was seen in Figm-e 71 to consist of eight cells, which have now in- 

 creased (Figure 78) to sixteen, and lie crowded together in two rows, one 

 superposed above and overhanging the other (cf. Figs. 79 and 81. 



In Figures 62 (Plate X.) and 71 (Plate XL) we saw that the two 

 posterior chorda cells, viz. c'-^^, d'"^^, were separated from the ante- 

 rior chorda cells by the mesenchyme cells, E^-^, A''-^, or their descend- 

 ants. In Figure 82 w^e see that the descendants of B'-^, A''^ (viz. Br'--^, 

 ^.22^ ^9.23^ jp.'ii^ a^Q(j ^\^Q corresponding cells in quadrant A) during the 

 process of invagination have been pushed down to the level of the other 

 mesenchyme cells, allowing the anterior chorda cells to come into contact 

 with the isolated posterior chorda cells (Fig. 81, c^-^\ c^-^'^) above them. 

 The posterior chorda cells were seen to be in the seventh geueratiou in 

 Figure 71 (c^'^^, d'''^^). At the stage represented in Figure 81, there is 

 good reason to believe that they have divided and passed into the eighth 

 generation, since every other cell of the dorsal hemisphere is known to 

 have done so previous to that stage; they are therefore represented by 

 the cells c^-^\ c^-^^, c?^-^\ and c?*-^^, the last named cell being hidden from 

 view in Figure 81 by the overlying muscle cell. 



The endoderm cells still remain in the eighth generation, and num- 

 ber twetity. Their arrangement is made clear by an examination of 

 Figures 81-83, in comparison with Figure 79, which represents a section 

 near and parallel to the median plane of a slightly earlier stage. Four- 

 teen of the twenty endoderm cells abut on the median plane, and six are 

 placed laterally toward the anterior end of the embryo. The median 

 double row of cells consists of 6^-" (Fig. 81), V-^^ (Fig. 82), i^.is^ jv.ie^ 

 ^7.16^ g7.H ^7.13 ^pjg_ §3^^ ^j-,^ ^j-jg (.orresponding cells in the right half of 

 the embryo. The nuclei do not appear in the centi-ally and posteriorly 

 situated endoderm cells of Figure 83 because they lie in later (deeper) 

 sections of the series, not figured (cf. Fig. 79). Only the narrow upper 

 ends of the cells in question appear in Figure 83, which therefore gives 

 no adequate idea of their size, but a correct idea of this may be had by 

 an examination of Figure 79. The laterally situated endoderm cells 

 are c'-^ (Fig. 82), c"'-^°, c'-^^ (Fig. 83), and the corresponding cells in 

 the right half of the embryo. 



From a series of cross sections through an embryo in about the same 



