362 TUNICATA. 



later stages which are connected with ;i lengthening of the caudal 

 section, the cells of the chorda-strand also lengthen (Figs. 167 A, 

 170, ch). The chorda then begins to undergo a transformation which, 

 at its commencement, is comparable to the changes in the chorda 

 of Amphioxus, but, in the Ascidians, leads to peculiar modifications in 

 this organ. Between each two consecutive cells there appears a 

 vacuole filled with a gelatinous substance (Fig. 167, v; cf. also Fig. 

 170). These vacuoles, which, at first, lie in the axis of the chorda- 

 strand, as they enlarge, compress the chorda cells in such a way that 

 the latter soon assume the biconcave form of fish- vertebrae and, as 

 the gelatinous mass extends further, can be recognised merely as 

 thin septa between its different sections. These sections soon come 

 into contact and fuse, and in this way a strand of homogeneous 

 gelatinous substance arises which at first resembles a string of beads 

 but is later uniform and cylindrical (Fig. 168 A), while the chorda- 

 cells which are pressed out to its surface surround it as a kind of 

 sheath (Kowalbvsky, Kupffer, Seeliger, and others). 



The transformation of the chorda is not, in all Ascidians, so complete. 

 According to Seeliger, in Clavelina, it does not advance beyond the stage in 

 which the chorda-cells assume the form of transverse septa. 



Mesoderm, body-cavity, musculature. The two mesoderm-bands 

 accompany the chorda along its whole length and project a little 

 beyond it anteriorly. Two parts can be distinguished in them (Figs. 

 162 B, 163 B). In the posterior part (ms) where they consist of a single 

 layer of large cells arranged in three longitudinal rows, they yield 

 the musculature of the larval tail. The cells of this part lengthen 

 in later stages of development (Fig. 168 />', m) becoming hexagonal, 

 while, on their inner and outer surfaces they produce longitudinal 

 fibrils of contractile substance (Fig. 169, nr.) which appear to lie 

 somewhat obliquely to the longitudinal axis of the body in such a 

 way that the fibres of the inner layer cross those of the outer layer 

 at an acute angle (Seeliger). The caudal musculature of the larva 

 which arises in this way shows indistinct transverse striation. 



Anteriorly, in the trunk-region, the mesoderm-bands consist of 

 several layers of smaller cells at first closely crowded together (Figs. 

 161 J, ms, 162, 1 ()."), ms). The innermost layer which lies next to 

 the chorda is evidently a direct prolongation of the myoblast-layer of 

 the caudal region. It undergoes the same transformations as the 

 latter and yields the anterior part of the larval musculature. The con- 

 nection between the other mesoderm-cells of this region soon becomes 



