560 WILLIAM BATESON. 
Now, returning to what is found in Balanoglossus, it is to be 
noted that, first, the cord separates from the skin as a solid 
rod connected at the two ends to the skin, and upon this con- 
dition invagination supervenes at the two ends, forming a 
neural tube in these regions. Let us follow the effect which 
an extension of this system of invagination along the cord will 
have upon the origin of the dorsal roots ; for it is nearly certain 
that invagination in this case is secondary to delamination ; the 
condition in Amphioxus, in which the medullary plate folds up 
after being enclosed, offering a stage of transition between the 
condition found in Balanoglossus and that of an Elasmobranch, 
for example. Since the invagination of a plate of tissue differs 
from the separation of a cord in the fact that it is not the 
central line, but the two edges of the plate, which remain last 
in connection with the skin, it follows that, as the process of 
invagination phylogenetically arrives at the point of attachment 
of any one of these median dorsal roots, it must take up its 
new attachment at one of these two edges. It is thus not 
possible, supposing these views correct, that the dorsal roots 
could in the first instance have been paired, except on the 
hypothesis that as the process of invagination phylogeneti- 
cally reached its point of attachment each dorsal root split into 
two; which is almost impossible, and which the condition of 
Amphioxus shows not to have occurred. ‘The other alternatives 
would be (1) that all the dorsal roots should remain attached 
on one side to the cord; (2) that they should be attached 
irregularly to one side or the other; and lastly (3) that 
they should have been attached alternately to either side. 
From the nature of the case they could not be opposite. 
Now, the fact of their alternate arrangement in Amphioxus 
is almost a proof that the latter alternative was the one 
which occurred. (It may be observed that, as a physio- 
logical convenience, they probably supplied the two sides 
of the body alternately while yet attached in the middle 
line.) Thus the opposite origin of the dorsal roots is 
almost certainly secondary to an alternate arrangement. 
The fact that it is the foremost pairs which are opposite in 
