CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 263 
the posterior portion of the neuro-museular ring ; those are destined to 
form the longitudinal musculature of the tail. The medullary plate, 
which produces the entire nervous system of the trunk region, lies wholly 
” of the lips of the blastopore. 
anterior to the region of “ concrescence’ 
5. The posterior margin of the blastopore does not grow forward over 
the blastopore covering in the medullary canal as deseribed by Van 
Beneden et Julin (86) in the case of Clavelina.! 
6. I heartily concur in Samassa's (94) conclusion that there is no 
rotation of axes during the gastrulation of Ciona, such as Korschelt u. 
Heider (93), on theoretical grounds, conjectured might occur in Ascid- 
ians. ‘Their hypothesis is, so far as I know, entirely unsupported by 
observation. 
4. Formation of the Larva. 
The further changes which the embryo undergoes in its transformation 
into the larval tadpole will be understood from an examination of Fig. 
ures 99-105 (Plate XIII), which represent seven sections through an 
embryo with completely closed blastopore. Figure 99 shows tho third 
section (in passing from behind forward) of the series; it contains about 
half a dozen muscle cells and four nerve cells, surrounded. by an epithe- 
lial layer of ectoderm, ‘The first section of the series shows merely the 
ectoderm cut tangentially ; the second contains six muscle cells sur- 
rounded by the ectoderm, but no nerve cells or chorda. The four nerve 
cells in Figure 99 show precisely the same arrangement as is found later 
in a cross section of the tail of the larva. (Sce the four cells at the right 
of ed. in the right portion of Fig. 100.) 
In Figure 100 (Plate XIII.) the number of nerve cells (seven) is seen 
to be increased, and the chorda makes its appearance as a group of seven 
cells ventral to the nerve cells. 
In Figure 101 (Plate XIII) the nervousand chorda fundaments 
appear about as in the section shown in Figure 100, but underneatlt the 
chorda is seen a group of four small mesoderm cells, the descendants of 
D's, 07 (Plate XII. Fig. 88), which have at last divided. Just ante- 
rior to them in the embryo (Figs. 102 and 103) extends the double row 
of caudal endoderm cells. As T have already suggested (page 262), the 
subchordal mesoderm cells (Fig. 101) probably have the same fate as 
! The authors mentioned were doubtless led into this mistaken interpretation by 
identifying as nerve cells the muscle cells which lie behind the blastopore at the 
time of its closure. (See their Figs. 1o, 1e, 2e, 8a, Pl. VIL These figures are 
Figs. 741 A, 741 B, 742 B, and 745 B, 
reproduced in Korschelt u. Heider's (09 
respectively.) 
