ех 
82 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
and uniformly along the mid-line as at the sides. Practically all investigators, 
who have studied the embryology of Amphzoxus or the ascidians are in agreement 
upon this point, and if concrescence occurs among the amphibians, as is claimed 
by some investigators, though denied by others, it can only be said that in this 
respect the amphibians are very different from these other classes. Тһе evidence 
that the amphibians do form such an exception is by no means conclusive, as Ziegler 
(1902) points out. | 
The question whether and to what extent there 1s an actual inrolling of cells 
from the outer to the inner layer in the closure of the blastopore is one which has 
been much discussed. In all three of these chordate classes an inrolling of cells at 
the margin of the blastopore has been repeatedly observed, but the relative number, 
the origin and the character of such cells are matters of dispute. Lwoff (1894) 
maintains that the entire dorsal lip of Amphzoxus, inner as well as outer layer, is 
formed from ectoderm cells which are inrolled. All of these inrolled cells he counts 
as ectoderm and consequently concludes that the chorda and mesoderm are of ecto- 
dermal origin. The invagination of the endoderm is, in his opinion, the real gastru- 
lation, whereas the turning in of the ectodermal cells is a coenogenetic process 
which has nothing to do with the formation of the enteron but is concerned only 
with the formation of chorda and mesoderm. This conclusion has been criticised 
by Samassa (1898), Klaatsch (1896), Morgan and Hazen (1900), e¢ а/., on the 
ground that there 18 no sufficient evidence that the inrolled cells are ectodermal. 
With this conclusion, when extended to the ascidians, I heartily agree. Here 
the cells which are inrolled at the anterior border of the blastopore are chorda 
cells which are yolk laden and resemble endoderm and not ectoderm. The cells 
which are inrolled at the posterior lateral borders are mesench yme and muscle cells 
and in histological structure are very unlike the ectoderm. While therefore agree- 
ing with Lwoff that the chorda and mesoderm cells are inrolled (though from 
opposite portions of the blastopore lip in ascidians) I agree with his critics that 
‘hese cells, judged by their lineage and histological character, are certainly not 
ectodermal. 
6. Neural Plate. 
In ascidians the neural plate material becomes segregated into six cells at the 
44—cell stage ; four of these cells lie in a transverse row at the anterior border of 
the dorsal hemisphere, just below the third cleavage plane and two of them lie 
just above this plane and therefore in the ventral hemisphere. Тһе four dorsal 
cells lie just anterior to the four chorda cells from which they were separated at the 
sixth cleavage. Both the dorsal and ventral cells divide transversely, the former 
giving rise to an arc of eight cells the latter to one of four cells, and to these a 
single additional cell is added on each side making an are of six neural plate cells 
in the ventral hemisphere, In subsequent divisions the neural plate increases much 
in length and its anterior portion also increases in breadth, but it is never more than 
eight cells wide in its posterior part. Soon after gastrulation begins the neural 
cells overgrow the chorda cells and thereafter cover the dorsal lip to its posterior 
