1897-98.] Dr Masterman on Archimeric Segmentation. 295 
to the axis of the gastrula and the closure of its intermediate 
portion to form a pair of apertures, the mouth and anus. 
In ontogeny, especially in larval forms, the assumption of the 
secondary axis of the gut is greatly hastened, so that the blasto- 
pore instead of becoming closed in this manner, becomes carried 
over to form the mouth or anus alone, the other aperture (anus or 
mouth respectively) being formed later by secondary invagination. 
The natural result of this acceleration is that, whilst the archen- 
teron is only just sufficiently differentiated to give off its ccelomic 
diverticula at the distal or aboral end, its main axis has already 
become parallel to the bilateral axis and the proximal end is 
already formed into the mouth or anus. If the former takes place, 
Fig. 17. — Horizontal section through 
embryo (typical) of Echinoderm 
to show formation of mesoderm. 
Blastopore forms anus. 
Fig. 18. — Horizontal section through 
embryo in which blastopore be- 
comes mouth, or in which it closes, 
and the stomodseum is early 
formed, as in Sagitta. 
then the coelomic pouches will arise from the posterior end of the 
gut as two diverticula, which will move forwards in the horizontal 
plane, and will give rise to paired rudiments of the protocoele, then 
the paired mesocoeles, and, finally, the metacoele (fig. 18). On the 
other hand, if the latter takes place, i.e ., the blastopore becomes 
precociously the anus alone, then the mesoderm will arise from the 
distal extremity as before as paired diverticula, which will move 
backwards and will give rise distally to paired metacoeles, paired 
mesocoeles, and, lastly, an unpaired protocoele (fig. 17). 
The former method of archicoele formation is apparently pur- 
sued in the Clujetognatha and the Mollusca [Patella) A and the latter 
in a great number of the Echinodermata. 
Lastly, if the blastopore closes completely, and the archenteron 
* Patten, W., Arb. Zool. Inst. Wien , 1886. 
