336 
E. W. MACBEIDE. 
an excellent reason can be given why we cannot expect to 
find a trace of the slit-like inoutb in Vertebrate ontogeny, 
for there is no trace of it in the development of Balano- 
glossus nor in the development of the Echinodermata, 
which is the group most nearly allied to Balanoglossus 
and the Enteropneusta. In both Enteropneusta and Echino- 
dermata the blastopore becomes the anus and the mouth 
is formed later as an independent meeting of ectoderm 
and endoderm. I am strongly inclined to believe that the 
“ seam ” which originally connected mouth and anus was 
situated on the ventral and not on the dorsal surface. It 
is quite possible tliat the second stage in gastrulation, viz. 
the upgrowth of the ventral lip and the coincident union of 
its lateral halves, may be a reminiscence of this closure. The 
great difficulty in fixing points of reference in Vertebrate 
development is that general growth in length supervenes so 
early that before the mouth is formed the embryo has altered 
in size and shape to a great extent. Nevertheless the place 
where the mouth is formed in Amphioxus cannot be far 
from the point y in the gastrula in fig. 6, in which the 
upgrowth of the ventral lip has not yet begun. 
Now Balanoglossus presents us with a condition where 
the nervous system, as in some Nemertinea and Echino- 
dermata, is practically co-terminous with the ectoderm — a 
condition therefore in which a central nervous system could 
originate anywhere where stimuli were concentrated. The 
local concentration in the dorsal region of the collar which 
we believe to be the forerunner of the earliest part of the 
A^ertebrate dorsal nerve-tube receives its explanation from 
the structure of the other Enteropneusta, Cephalodiscus and 
Rhabdopleu ra. In both of these animals the collar region is 
produced into a series of arm-like outgrowths beset with 
ciliated tentacles. The dorsal nerve cord is a centre co- 
ordinating the two halves of this apparatus. The free- 
swimming pelagic ancestor of Enteropneusta probably 
possessed similar developments of the collar region, and it is 
interesting to reflect that the ciliated cirri” which fringe 
o o 
