119 
especially is never fulfilled, the anus always arising as 
an independent perforation. Peripatus only could be 
adduced here as a support to SEDGWICK's view, the 
same form which also inspired SEDGWICK himself to his 
theory. If, however, the still insufficiently elucidated proces- 
ses of the early development of Peripatus have been rightly 
interpreted by their investigators, we can only state that 
this form holds in this respect a quite exceptional position. 
BALFOUR's (1881, IL, p. 308, 317) opinion seems to me much 
better founded; he views in the Pilidium the larval form 
which most nearly approaches the characters of the radiate 
larval prototype in the course of its conversion into a 
bilateral form, the latter being reached by the unequal 
elongation of the oral face, the aboral dome forming the 
praeoral lobe and the oral half growing out into the seg- 
mented trunk. Already in the Pilidium the archenteric 
pouch is directed backwards and evidently it has broken 
through in the trochophora, as we see in ontogeny, thus 
forming an anal aperture which has nothing to do with 
the mouth or the blastopore. 
As regards the application of SEDGWICK's principle to 
Vertebrates, we must state in the first place that, unlike 
in Annelids, there is here no question of a relation of the 
mouth to the anterior end of the blastopore, while on the 
contrary the anus often shows certain connections to the 
posterior end. We shall see, however, at the end of this 
chapter that this connection is not of primary but of 
secondary nature. 
Experiments. — The theory of concrescence has found 
several adherents but still more numerous opponents. 
According to this theory we might expect to find a little 
incision in the middie of the anterior blastopore-border, 
and a raphe together with a coherence of ecto- and entoderm 
at least along a little distance in front of it, as a consequence 
of the coherence of ecto- and entoderm at the blastopore- 
border. Several authors have emphasized that of all this 
very little or nothing is to be recognized during the gas- 
trulation, especially in the lower forms like Amphioxus or 
Amphibia. On the contrary, we get much more the impres- 
sion that the blastopore closes by, truly eccentric but yet 
all-sided, contraction of its border over the yolk. Experi- 
ments made by MORGAN (1895), KOPSCH (1896) and 
SUMMER (1904) on fish eggs, the same objects which 
