296 Proceedings of Royal Society of Edinburgh. [sess. 
loses all connection with the epiblast, then it will not he affected 
by the distorted sequence of events, and will give rise to the archi- 
coeles in the primitive manner as separate diverticula. The same 
result will occur if the blastopore forms neither mouth nor anus, 
and in each of these cases there is apparently no essential reason 
why the archicoeles should not arise in quite as typical a manner 
as in the phyletic ontogeny, in which the blastopore becomes both 
mouth and anus. 
The facts again agree with this deduction, for in Balanoglossus, 
which has been cited as a form in which the archicoeles arise in a 
primitively independent manner, the arclienteron loses all connec- 
Fig. 16. — Horizontal section 
through embryo of Balano- 
glossus to show formation 
of mesoderm. 
Fig. 21. — Section through Ant&don 
embryo to show formation of 
mesoderm. (The mesodermic 
pouches are represented as syn- 
chronous in origin.) 
tion with the epiblast. Again, in Anted, on the archenteron 
becomes a free vesicle in the interior of the embryo,* and here there 
is a formation of the archicoeles, which is usually regarded as 
of a modified type, but which appears to me to be not only 
closely comparable to that of Balanoglossus but to be primitive. 
With the exception of the absence of a right mesocoele (right 
hydrocoele) in Antedon the types are identical. It is scarcely 
necessary to point out that any difference in point of time in the 
formation of the protocoele, mesocoeles, or metacoeles is immaterial, 
although the synchronous formation is probably the more primi- 
tive. Thus, in this case the archenteric vesicle first gives off the 
* Bury, H., Phil. Trans. Roy. Soc. Bond., vol. clxxix. 
