168 ■ T HE ANCESTRY OF VERTEBRATES 



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, II, 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 middle 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 



