THE ANCESTRY OF THE CHORDATA. 569 
(3) The degree of the invagination of its nervous system 
and the extent of the neural tube. 
(4) The extent and degree of isolation of its notochord. 
(5) The extent of the atrial folds. 
(6) The absence in B. Kowalevskii of any definite liver 
sacculi, and the presence in B. minutus, &c., of liver saccules 
differing from those of Amphioxus. 
The points of resemblance taken together are so consider- 
able as to suggest that they were possessed by a common 
ancestor of the Hemichordata and Cephalochorda. On the 
other hand, the points of difference are nearly all differences of 
degree, and (1), (2), (8), (4), (6) are points in which the 
Vertebrata agree with Amphioxus. In the case of (5), how- 
ever, the Vertebrata more nearly agree with Balanoglossus. 
Of the Vertebrata.—The common ancestor, then, of the 
Cephalochorda and the Vertebrata may be presumed to have 
possessed the features of mesoblastic repetition, invaginated 
nerve-cord, and consequent extension of the neural tube, raised, 
so to speak, to the degree in which they are found in both those 
divisions. Also it may be believed that the preoral lobe had 
somewhat diminished and that the atrial folds were still small. 
The origin of such a liver as that of Amphioxus, as a speciali- 
sation of part of the wall of the digestive region of a young 
B. Kowalevskii is easy to imagine, for the histology of these 
two tissues is still almost identical. [The presence of peculiar 
liver saccules in B, minutus, &c., presents no difficulties, as 
their absence in the more primitive B. Kowalevskii shows 
that they have arisen within the limits of the group.] Animals 
possessing those features would answer nearly to the Proto- 
chordata of Balfour, though the structures now attributed to 
it are somewhat different. 
The Protochordata thus constituted would then differ from 
the Enteropneusta in the possession of a serially-repeated me- 
soblast, in addition to serially-repeated gill-slits, and possibly 
generative organs; also in the complete separation of the 
nervous system and notochord. The serial repetition of the 
gill-shts, the small operculum, &c., they must be presumed to 
