ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 83 
margin. During all this time the anterior margin of the plate reaches only about 
one-third of the way from the equator to the animal pole. The posterior margin of 
the plate reaches nearly to the hinder end of the embryo, and when the blastopore 
closes a pair of V-shaped folds runs forward from the region of the blastopore 
inclosing the neural plate between them. These neural folds then fuse from behind 
forwards thus converting the plate into a tube. Dorsal to the notochord the neural 
tube becomes solid; in the region in front of the notochord it retains its lumen. 
There is no nerve ring around the blastopore and probably none of the ecto- 
derm cells around the posterior margin of the blastopore are added to the neural 
plate. 
In Amphioxus and amphibians the neural plate is first recognizable about the 
the time of the closure of the blastopore. As in ascidians it arises in the outer layer 
of the dorsal lip and extends back as far as the blastopore, but whether its cells 
arise in close connection with the chorda and from both dorsal and ventral hemi- 
spheres as in the ascidians is unknown; furthermore, the distance of the anterior 
edge of the plate from the animal pole is unknown. Тһе work of Kopsch (1900) 
indicates that in the frog the anterior margin of the plate is situated less than 
half the distance from the equator to the animal pole, and H. V. Wilson (1900) in 
particular has shown that the anterior part of the neural plate is formed from the 
black hemisphere, the posterior part from the white hemisphere,—a result which 
agrees precisely with my observations on ascidians. As is well known the method 
of closure of the neural tube in Amphzoxus is peculiar, while the solid character 
of the hinder part of the tube is peculiar to the ascidians, but with these exceptions 
the later history of the neural plate and tube is essentially similar in all three 
classes. 
7. Chorda. 
In ascidians the substance of the chorda is segregated into a single trans- 
verse row of cells just posterior to the neural cells at the 44-сеП stage, before 
there is a trace of gastrulation. These chorda cells are generally clearer and 
contain rather less yolk than the endoderm cells which lie immediately posterior to 
them. These four chorda cells divide transversely forming an arc of eight cells and 
soon thereafter a depression of the endoderm occurs posterior to this are, which is 
the beginning of the gastrulation. These chorda cells are flanked on each side by 
the most anterior cells of the mesenchyme are, the two arcs together forming the 
chorda-mesenchyme ring of Castle. The eight chorda cells then divide antero- 
posteriorly forming two rows of eight cells each. This plate of cells by shoving, 
by interdigitation and perhaps to a limited extent by folding, decreases in width and 
` Increases in length, the cells finally, in a late larval stage, becoming arranged in a 
single linear series. When they first arise the chorda cells are superficial in posi- 
tion, but in the overgrowth of the dorsal lip they are inrolled so as to lie in the 
roof of the gastrocoel. Тһе posterior growth of the dorsal lip carries the entire 
chorda into the hinder half of the embryo, and it afterwards extends to the tip of 
the developing tail. 
