22 
W. H. CALDWELL. 
derived from the primitive mouth.” I fail to see how his theory 
on p. 34 explains the behaviour of the blastopore. In the first 
place his conception of the blastopore is different from that used 
in the present paper. Page 35 be writes, “ Consequently the only 
course open is that the mouth should be formed as a secondary 
perforation entirely independent of the blastopore.” Sedg- 
wick’s theory is contained in the following passages (p. 34) : 
“ My view is that in those animals in which it does not give 
rise to the mouth and anus, it functioned as the larval mouth 
while the animal was developing, and persisted until parts of 
the embryo were developed between it and the position of the 
mouth and anus of the adult, which parts had arisen in the 
phylogenetic history in the adult after the primitive mouth had 
completely divided into the mouth and anus. These parts 
never had been traversed by the original slit-like mouth, 
because they had appeared at a stage in evolution subsequent 
to the stage in which the mouth and anus were one. It cannot 
therefore be a matter of surprise if the blastopore does not 
elongate and bisect these latter structures, which never had in 
the history of the animal been perforated by the blastopore.” 
I ask, how have the cells which are to form mouth and 
anus anything to do with blastopore ? My hypothesis is as 
follows. 1 
The behaviour of the blastopore in Phoronis is obviously due 
to the attainment of a terminal anus. Suppose the long axis 
of the body to increase still more rapidly while the posterior 
part of the blastopore still remains terminal. Suppose in the 
early stages of development the importance of a complete 
alimentary canal is not equal to the importance of the body 
form, then the tendency of the endoblast to divide into anterior 
and posterior portions attached to anterior and posterior parts 
of the blastopore respectively might be consummated. The 
behaviour of the cells in the middle of the primitive streak of 
Phoronis, which resulted in the opening of a space between 
endoderm and ectoderm, would tend to begin at an earlier stage. 
1 Delamination need not be discussed, since the existing hypotheses (vide 
Balfour, * Embryology,’ vol. ii) are sufficient to bring the case uuder my theory. 
