Origin of Mouth and Anus 57 



dorsal portion alone continues, with the result that a tail is formed. 

 This I have endeavoured to show is the only conclusion to which 

 we can come as the result of experimental and anatomical 

 observation in the chordate embryos. Therefore the relation of 

 the main axis of the Vertebrata is, as I have indicated, at right 

 angles to the plane of the blastopore in Fig. 31 C. 



This conclusion is diametrically opposed to the conclusion of 

 His, Hubrecht, Dohrn, Semper, Sedgwick and others, who derive 

 the dorsal surface of the chordate from the coalescence of an 

 elongated blastopore surface. 



So even MacBride, who after fully accepting as true the views 

 of growth in length as I have indicated them, comes to the con- 

 clusion, that "the mouth and anus in all animals in which two 

 such openings are found, owe their origin to the division of a long 

 slit-like coelenterate mouth," and suggests that the seam which 

 originally connected mouth and anus was situated " on the ventral 

 and not on the dorsal surface." 



This supposition has not a trace of embryological evidence in 

 its favour. 



Goodale's experiments already alluded to show that there is 

 no suggestion of concrescence either along the ventral or dorsal 

 border connecting mouth and anus. 



MacBride suggests that the growth upwards of the ventral lip 

 of the blastopore in Amphioxus is reminiscent of this concrescence. 

 It is true that a similar and more obvious coalescence of the 

 ventral part of the blastopore lip occurs in the Amphibia, but the 

 most ventral part either remains as anus or re-opens as anus 

 never is there any sign of connection with the mouth. We might 

 legitimately regard the Amphibian condition as representing an 

 ancestral coelenterate or actinian phase in which the neurenteric 

 end or opening represents the actinian inhalent channel; whilst 

 the anal end or opening represents the exhalent channel. 



One may argue thus. When the chordate central nervous 

 system began to become bulky and in forming a tube involved 

 the inhalent channel (as neurenteric channel), then one or more 

 of many other openings which in no way owed their origin to the 

 blastopore became an inhalent aperture or mouth. There is no 

 difficulty in assuming the adoption of a new opening for the mouth 



