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 cud of the embryo, and when the blastopore 

 doses 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. The 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 (1000) 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 Amphioxus 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-cell 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 arc. which is 

 the beginning of the gastrulation. These chorda cells are flanked on each side by 

 the most anterior cells of the mesenchyme arc. 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. The 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. 



