castle: embeyology of ciona intestinalis. 253 



position ill the dorsal wall of the archenterou ; we may now observe that 

 certain cells of the latero-posterior segments of the neuro-muscular rin"- 

 are also involved in this ingrowth or invagination. At the stage shown 

 in Figures 74-76 there was a double row of neuro-muscular cells on 

 each side of the blastopore, now (see Fig. 80) the inner row is nearly 

 covered from sight by the row of cells outside it. 



Already at the stage shown in Figure 72 the most posterior neuro- 

 muscular cells (Z)^•^ C^-^) had moved from their originally lateral posi- 

 tion toward the median plane (cf. Fig. 71). There they were destined 

 presently to meet each other, covering over the small mesenchyme cells 

 D'-^, C-^ (cf.Figs. 71 and 72 with Fig. 80) ; now (Fig. 80) they or their 

 descendants lie at the posterior angle of the blastopore, and are in turn 

 being covered over by the more laterally and anteriorly situated neuro- 

 muscular cells. 



The nerve cells anterior to the blastopore have increased considerably 

 in number, perhaps through additions from the ectodermal group (cf. 

 Figs. 72 and 80, also Figs. 78 and 79). 



Three sections from a horizontal series through an embryo of about 

 the stage shown in Figure 80 are represented in Plate XI. Figs. 81-83. 

 The series consists of thirteen sections 6.G7jtt thick, of which Figure 

 81 represents the third, Figure 82 the fifth, and Figure 83 the seventh. 

 The sections are a little oblique, and consequently strike the right and 

 left halves of the embryo at slightly different levels. The left side of 

 Figure 82 shows best the history of the mesenchyme cells since the last 

 stage examined in detail (Figs. 73-77). Lateral to the small gastral 

 cavity we find the sister cells B^-^^, B^-^*, descendants of the common 

 mother cell B^-'^^ (Fig. 77). Evidence of the derivation of these two cells 

 has been cited in the observation of a spindle longitudinally directed in 

 the cell B^-^^ in two different embryos less advanced than tliis. 



Lateral to B^-^^ and B^--* are the sister cells B^-^\ B^-^\ descendants 

 of 5^-11 (Fig. 71 ; cf. A^-^\ Fig. 77). They stain more faintly than 

 B^-"^^ and 5®-^*, a distinction which, it will be remembered, existed 

 between the respective mother cells i?^-" and B^"^^ (Fig. 77). Though a 

 spindle has in no embryo been directly observed in ^•", evidence of the 

 sistership of B''^^ and B^-^^ (Fig. 82) exists in the still persistent inter- 

 zonal filaments which stretch between their nuclei. This evidence is 

 supported by the similarity of the cells in size and stainability. Poste- 

 rior to the quartette of cells just discussed, the common descendants of 

 B''-^ (Plate X. Figs. 62, 67), are the two daughter cells derived from 

 c''-i2 (Yxg. 71), which was seen to be mitotic at an earlier stage (Fig. 75). 



