20 
W. H. CALDWELL. 
Anus. 
Meanwhile the remnant of the primitive streak, the posterior 
solid cord of cells, opens up, and forms a canal leading from 
the archenteron to the exterior (fig. 15). The alimentary 
canal is now complete, mouth and anus having been derived 
from the blastopore (fig. 7, m and a). 
Hypothesis. 
With the help of the various morphological laws implied 
by such terms as precocious segregation, superlarvation, abbre- 
viation, &c., it is possible to solve almost any morphological 
problem in several ways — all equally probable. The speculations 
which follow, I am induced to add to those already existing, 
not from any belief in their absolute value, but because they 
go in the direction of simplification. The theory which I am 
about to state reduces the various origins of the mouth and 
anus to one type. The same hypothesis gives an explanation 
of the various modes of origin of the mesoderm, and leads to a 
view of the meaning of metameric segmentation which, so far 
as I know, has not been hitherto suggested. 
Given a gastrsea already become bilaterally symmetrical by 
the elongation of the blastopore and the differentiation of 
anterior inhalent and posterior exhalent currents, and in which 
the main development of organs takes place around the mouth, 
so that the mesoderm thus resulting comes to lie in develop- 
ment as two masses of cells on either side of the body. 1 
I propose to show that the elongation of a long axis of the 
body is a possible cause of — 
I. The obliteration of the relation of blastopore to mouth 
and anus. 
II. The masking of the original mode of mesodermic forma- 
tion. 
III. Metameric segmentation. 
1 Whether the mesoderm originally arose as diverticula or not does not 
concern the present speculation. 
