22 



of Annelids is absent in Vertebrates, and that the brain and spinal 

 cord of the latter may be compared directly with the ventral cord 

 of Annelids, then a whole host of direct structural relationships between 

 Annelida and Vertebrates may be established. 



Kleinenberg expresses his opinion that the spinal ganglia of 

 Vertebrates have their parallel in the parapodial ganglia of Annelids, 

 — a comparison which, as I shall elsewhere show, is entirely justi- 

 fiable, from the development, for the spinal ganglia and for certain 

 portions of the cranial ganglia as well. 



To make the present notice as short as possible, let Ime now 

 briefly review the conditions we must have in any structures of 

 Vertebrates, which are to be homologised with the permanent mouth 

 of Annelids. 



The corresponding structure in Vertebrates ought to arise as a 

 paired involution of epiblast (though it is conceivable a priori that 

 the paired character might be lost). This involution must fuse with, 

 and open into the cavity of the hypoblast. It must also give rise to 

 certain glands, and it must have a special nervous system of its own 

 derived from the hinder part of the first ventral ganglion or its 

 homologue. This nervous system must supply it alone, and no other 

 part of the alimentary canal. 



All these conditions are fulfilled hy the complex called hypo- 

 2)hysis cerebri. 



In at least one case (Hippocampus) the oral hypophysis ^) is 

 known to arise as a paired involution (Dohrn). In the Cyclostomata 

 it is formed as an epiblastic involution (possibly paired) at the extreme 

 anterior end of the body. In one Vertebrate alone, Myxine, it still 

 opens into the hypoblast; in all others it approaches the hypoblast in 

 development, but does not fuse with that layer. It always lies in 

 very close relationship with the extreme end of the notochord; that 

 is, with the end of a structure derived from the hypoblast. In 

 adult Petromyzon, in which the tube of the oral hypophysis has the 

 same relationships as in Myxine , except that the posterior opening 

 into the hypoblastic sac is absent, it nevertheless has an astonishing 

 length, and ends blindly very close to the gut. In Myxine and Petro- 

 myzon tubular glands are developed in connection with it. In all 

 the higher Vertebrates, in which the oral part is very rudimentary, 

 it always has a distinct glandular character. 



1) I believe it is very .frequenlly paired , though not at its point 

 of origin. 



