108 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
30° from the equator to a point a little below the equator (text fig. XX XIX); no 
portion of the nervous system comes from the region of the animal pole and none 
from the posterior pole. Furthermore, the mouth, which is here a new formation 
and has nothing to do with the blastopore, does not open through the nerve plate 
but lies between the anterior end of the neural plate and the animal pole. 
We have here differences of a fundamental order, even in the earliest stages of 
development, between the vertebrate, or rather chordate, and the invertebrate ; the 
early development throws no light upon the way in which the one may have been de- 
rived from the other. It is of course possible to conceive of a condition in which the 
nervous system surrounded the entire blastopore as a ring, which in the case of the 
annelids underwent concrescence from behind forward, thus forming the ventral 
plate and ganglia, but which in the chordates underwent concrescence from in front 
c.g. 
end. 
end. 
XXXIX XL 
Fies. XXXIX, XL.—Diagramatic representations of the types of germinal localization in ascidians 
and annelids. Mesodermal substance is shaded by lines, neural substance by fine stipples, and chorda 
(end. )a are di fia as ама represented, at the close of the first cleavage. Fig. XL, the annelid ty 
1 Th 
are not ditio puie in the unsegmented egg, but are shown in the regions to which they may be 
t y means of the cell lineage. 
backwards. But however probable such a theory may be it finds little support in 
the early development of ascidians. It is true that a nerve ring has been described 
as surrounding the blastopore in ascidians, but I have not been able to find evidence 
of its existence. Furthermore, there is no evidence in the development of ascidians 
that there is any concrescence of the anterior lip of the blastopore ; en the contrary 
the anterior lip grows backward over the archenteron as rapidly in the mid-line as 
at the sides,—a view in which practically all writers on ascidian embryology agree. 
Finally, the lack of an apical plate and cerebral ganglion at the animal pole in the 
ascidian constitutes a notable difference from the condition found in most inver- 
tebrates. In his great work on Sa/pa, Brooks (1893) has shown in masterly fashion 
