570 WILLIAM BATESON. 
have acquired from the ancestor common to them and the 
Enteropneusta. 
In this way the connection of the Protovertebrata of Balfour 
with the other division becomes explicable on the new facts 
derived from the Enteropneusta. 
The peculiar fact that so many of the features of the 
Enteropneusta differ from those of the Cephalochorda in 
degree of expression only is very remarkable, and suggest 
that their further evolution towards the Protochordate type 
proceeded by correlated variations affecting the several 
systems. 
From the Protovertebrata thus constituted, which in all 
probability possessed an unsegmented mesoblastic sheath for 
the notochord and a brain, the Cyclostomata may be easily 
derived without the necessity of any hypothesis of great 
degeneration, which cannot be well supported. 
Balfour has fully discussed the question of the origin of his 
hypothetical group of Protognathostomata, and upon the 
question of their immediate origin no new light can be thrown. 
The above suggestions entail many difficulties. The chief of 
these is that they involve the hypothesis that the rudiment 
of the notochord of the Archichordata developed itself 
as -a separate structure, once in the case of the Ascidians, and 
again in the case of the Protochordata. In the first case, 
owing to the atrophy of the przoral lobe and use of the tail in 
swimming, it came to lie in that organ, and in the second case 
extended through the whole length of the body. Also does this 
suggestion of the origin of the Tunicates involve the proposition 
that the rudiment of the dorsal nerve-cord extended itself 
twice along the body, once in the case of the Ascidians, and 
again in the case of the Protochordata. If this occurred there 
is no difficulty in supposing it to have been twice invaginated, 
this being a more less common feature among nervous 
systems. 
Another difficulty which affects all these suggestions arises 
from the epiblastic origin of the generative organs of Enterop- 
neusta, in which they resemble the Echinoderms. 
