CASTLE: EMBKYOLOGY OF CIONA INTESTINALIS. 263 



the posterior portion of the nearo-muscular ring ; these 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 

 anterior to the region of " concrescence " of the lips of the blastopore. 



5. The posterior margin of the blastopore does not grow forward over 

 the blastopore covering in the medullary canal as described 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- 

 iaus. Their hypothesis is, so far as I know, entirely unsupported by 

 observation. 



4. Formation of the Larva. 



The further changes which the embi'yo 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 the 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 Figui-e 99 show precisely the same arrangement as is found later 

 in a cross section of the tail of the larva. (See the four cells at the right 

 oi cd. in the right portion of Fig. lOG.) 



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 nervous and chorda fundaments 

 appear about as in the section shown in Figure 100, but underneath the 

 chorda is seen a group of four small mesoderm cells, the descendants of 

 jy,.5 Q1.5 (Plate XII. Fig. 88), which have at last divided. Just ante- 

 rior to them in the embryo (Figs. 102 and 10.3) extends the double row 

 of caudal endoderm cells. As I have already suggested (page 262), the 

 subchoi'dal mesoderm cells (Fig. 101) probably have the same fate as 



1 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. la. Ic, 2c. 3n, PI. VII. These figures are 

 reproduced in Korschelt u. Heider's ('93) Figs. 741 A, 741 B, 742 B, and 745 B, 

 respectively.) 



