‚84 ORGANIZATION AND CELL-LINEAGE ОЕ ASCIDIAN EGG. 
In Amphioxus the earliest stage at which the chorda has been positively iden- 
tified is one when the blastopore is small and the embryo elongated. According to 
Hatschek it consists at this stage of a plate, about six cells wide, in the roof of the 
archenteron and extending along the mid-line of the dorsal lip throughout its entire 
length. This plate is narrower and longer than it is in the ascidians, but is other- 
wise much the same in appearance. The later history of the chorda is essentially 
the same in both forms. With regard to the origin of the chorda cells in 
Amphioxus, Morgan and Hazen (1900) have shown that the cells which are inrolled 
in the formation of the dorsal lip and some of which must take part in the forma- 
ation of the chorda, are clear and contain less yolk than the endoderm cells. 
Whether these cells form at this stage a plate which is wider from side to side 
than it is long, as is true of the ascidian, is not known. Lwoff (1894) has also 
recognized the fact.that the chorda cells are rolled in at the margin of the dorsal lip, 
and for that reason he regards them as of ectodermal origin. 
In Amphzoxus and in some amphibians the definitive roof of the enteron arises 
from cells which lie along each side of the chorda plate, and which finally grow 
under that structure and thus separate it from the gastric cavity ; in the ascidians the 
chorda lies ultimately in the posterior part of the body where the gastric cavity 18 
almost entirely lacking and there is no growth of endoderm cells under it to form the 
roof of the enteron. In most amphibians the chorda does not form a broad plate of 
cells, but is a narrow rod closely united ventrally with the endoderm, which forms 
the roof of the enteron, and connected laterally with the mesoderm. Іп these 
three groups of chordates the chorda plate is widest in ascidians and narrowest 
in amphibians. In all three it lies in the dorsal lip and is connected laterally 
with mesoderm (text figs. XXXVI—XXXVIII). Тһе later history of the chorda 
is essentially the same in all three classes. 
The question whether the chorda is of endodermal or of mesodermal origin is, 
as has been frequently said, one of definition of terms. Castle concludes that it is 
mesodermal because in Amphzoxus and lower vertebrates it “is derived from a 
common fundament with what is universally regarded as mesoderm” and also 
because it “comes to occupy a position between the inner and outer layers of the 
embryo." Оп the other hand, the histological structure of the chorda cells in 
Cynthia and Crona is much more like endoderm than mesoderm, and they are 
unquestionably derived from cells of the gastric endoderm at the 52-cell stage (fig. 
117, 193). I believe that special importance should attach to the structure of the 
cells which form the chorda, and if this be accepted as a guide the chorda, at least 
among ascidians, should be regarded as endodermal. 
8. Origin of Mesoderm. 
The exact place and manner of origin of the mesoderm of ascidians can be 
recognized with the greatest certainty in the gastrula, cleavage stages and even in 
the unsegmented egg. Тһе crescent, from which most if not all of the mesoderm 
arises, lies just below the equator of the unsegmented egg, and on the posterior 
