24 
W. H. CALDWELL. 
tion between these three methods, though obvious in Phoronis, 
has not been explained in the same way in the rest of the 
Triploblastica. The six diagrams (fig. 19) represent six dif- 
ferent modes of formation ; they may all coexist in the same 
animal. Thus, in Phoronis we find those represented in the 
diagram by 3, 4, and 6, and in the Chick and other Verte- 
brates 4 and 6. 
III. Metameric Segmentation. 
In Phoronis the elongation of the blastopore produces two 
pairs of masses of mesoblast, each of which might be re- 
garded as constituting a “ mesodermic somite but in Phoronis 
the first long axis developed is not to be the long axis of the 
adult. The long axis of an adult Phoronis is exactly at right 
angles to that of the larva. The further slight extension of 
the larval long axis is thus able to proceed pari passu with 
the growth of the posterior pair of mesodermic pouches. In 
Chsetopoda, Arthropoda, and Vertebrata the long axes of adult 
and larva are identical. The elongation of the body in these 
forms takes place before the mesoderm grows. The same 
cause which separated the mesoderm in Phoronis operates 
during a much longer developmental period. The mesoderm 
cannot keep pace with the ectoderm. It must therefore be 
left to afterwards complete its growth. The various positions 
in which it may remain give rise to the vai’ious origins of the 
mesoderm. Take Amphioxus, where the mesoderm has remained 
entirely in the endoblast. Here we have a regular elongation 
of the body taking place when the mesodermic cells are still 
undifferentiated. The mesodermic diverticula are regularly 
drawn out, and as regularly they leave small portions of the 
whole in front. Hatschek has described a shallow groove 
connecting the separated diverticula on each side, which is 
explained by the present hypothesis. I take it to be of the 
same nature as the connecting strand of mesoderm in Phoronis. 
Echiurus (Hatschek) is a form where the greater part of the 
mesoderm remains near the posterior pole of the long axis as in 
Phoronis. As the long axis grows the mesoblast has to be left 
