CASTLE : EMBKYOLOGY OF CIONA INTESTINALIS. 265 



In the region shown in Figure 105, it fills the entire interior of the 

 section. 



In Figure 106 (Plate XIII.) is shown a section through an embryo 

 in which the tail is already recognizable as a distinct portion of the 

 embryo, though it has not yet reached anything like its maximum length. 



It is curved veutrad uuder the trunk, so that the section passes 

 transvei'sely through both trunk and tail. The section passes through 

 the trunk in the brain region, but intersects only one of the mesenchyme 

 bands, the other one not extending so far forward in the embryo. The 

 endoderm cells are seen to have arranged themselves round a potential 

 lumen in the form of an epithelium. How^ever, they still lie two deep 

 in places. Their shape is clearly becoming cohmmar. 



In the tail region appears the chorda, now transformed into a single 

 row of flattened, disk-shaped cells, rapidly becoming vacuolated. They 

 form an axial rod extending through the entire tail region and the pos- 

 terior portion of the trunk. Dorsal (right in the Figure) to the chorda 

 lies the nei've cord of the tail, composed in cross section of about four 

 small cells. 



Ventral to the chorda is the sub-chordal endoderm strand consisting 

 of a double row of cells (en'drm.). On each side of the chorda are seen 

 in the section about three muscle cells. 



Summary on Formation of the Larva. 



1. The nerve cord in the limited I'egion of concrescence of the lips of 

 the blastopore is covered over by the ectoderm first at its posterior end 

 and then successively in its more anterior regions, following the course 

 of concrescence. The nerve cells in that portion of the embryo never 

 form a real canal, but only a potential one. They are arranged in a 

 solid strand, which usually shows in cross section four cells placed round 

 a common centre, the potential canal. 



The medullary plate arises wholly anterior to the blastopore. At the 

 time wdien the blastopore is about to close, the medullary plate has come 

 to extend over a great part of the length of the embryo, and has sunk 

 down in the form of a shallow grove deepest at its posterior end, the 

 anterior margin of the blastopore. When the blastopore closes, it be- 

 gins to form a canal. This process, like the fusion of the lateral mar- 

 gins of the blastopore, advances from behind forw\ard. 



2. Beginning shortly before the closure of the blastopore, a rapid 

 elongation of the embryo takes place, accompanied by a considerable 

 change ni its form and a reaiTangement of the cells composing some of 



vol.. xxvii. — xo. 7. 5 ^ 



