26 
W. H. CALDWELL. 
1. A gastrsea with slit-like mouth and a pair of lateral 
diverticula giving rise to mesoderm was the ancestor of 
Phoronis. 
2. The rapid growth of ectoderm in the median ventral line 
nearly succeeded in destroying the continuity of the primitive 
streak. 
3. The necessity of an early attainment of a terminal position 
by the anus caused the ectoderm to grow more rapidly than 
the endoblast, and resulted in a division of the mesoderm into 
anterior and posterior parts. 
4. The nephridia, which might have remained either wholly 
or in part with the anterior, have attached themselves entirely 
to the posterior mesoderm. 
Extension of this interpretation to the other Triploblastica — 
1. Phoronis is the first step towards a complete division of 
the blastopore. The inducing cause of such division is the 
elongation of the body, while the endoblast is still in an 
embryonic condition. 
2. The division of blastopore caused the division of meso- 
derm. 
3. The division of mesoderm results in — 
i. The masking of the original mode of mesoderm forma- 
tion. 
ii. Metameric segmentation. 
In Camp, Burnett River, 
Queensland \ July 27, 1884. 
