1897 - 98 .] Dr Masterman on Ar chimeric Segmentation. 301 
externally, but in Amphioxus they have become no longer defin- 
able, except in their mesodermic elements. 
Owing probably to the fact that Amphioxus has degenerated to 
a considerable extent, especially in the nervous system, one can 
find more traces of the archimeric segmentation in the Holoeliorda or 
Vertebrata. Here the neural tube, very typically and very early, 
shows a segmentation into three primary vesicles, the explanation 
of which has been obscure. In the light of the theory here put for- 
ward, the fore-brain corresponds to the protomere, and is evolved 
from the protomeric ectoderm. Such being the case, it is not surpris- 
ing to find that its actual wall gives rise to sense-organs (olfactory and 
optic), the protomere of the Archi-coelomata being typically sensory. 
Again, the mid-brain corresponds to the mesomeric or collar 
nervous ganglion, with its two nerves (III. and IV. cranial), supply- 
ing, respectively, the protocoele (pre-mandibular somite) and the 
mesocoele (mandibular somite), just as nerves direct from the 
collar-ganglion or archimeric central nervous system supply the 
protocoele and mesocoele in Actinotrocha Balanoglossus (muscles 
of proboscis and post-oral ring). 
Posterior to this is the elongated nervous tube or neurochord 
with its front end forming the hind-brain. This corresponds with 
the greatly specialised dorsal ectoderm of the metamere, a subordi- 
nate part of the body in Ar chi- chorda supplied with simple chords, 
but forming almost the whole of the body in the Eu-chorda. 
These relationships are even better brought out by a comparison 
of other organs in Actinotrocha and in the lowest Eu-chorda. This 
comparison is justified, because Cephalodiscus, Phoronis, Balano- 
glossus , Tunicata , and Amphioxus all have either sedentary or 
burrowing habits, and just to the extent that they are adapted for 
these habits do they differ in structure from the archi- vertebrate, 
wdiich, at least for the greater part of its history, must have been 
pelagic from Actinotrocha onwards. 
This cannot be further followed out here, but enough has been 
written to show that there is good evidence for believing that the 
Eu-chorda have been derived from the Ar chi-chorda by secondary 
segmentation of the third archimeric segment or metamere and 
gradual atrophy of the protomere and mesomere, as closed ccelomic 
pouches, though their walls persist as muscles. 
