324 A. T. MASTERMAN. 



The notochord must be regarded as arising in the collar- 

 region, and the prolongation forward of this organ in Am- 

 phioxus is usually acknowledged to be a secondary adapta- 

 tion, as is also the pre-oral position of Balanoglossus. 



The true Vertebrata, primarily free from a burrowing habit, 

 have probably retained at least the front end of the notochord 

 in its primitive position. The front end of the notochord, 

 therefore, indicating the anterior collar-region, the hypo- 

 physis is in the same relation to it as the subneural gland in 

 Actinotrocha. In the former case its inner end comes into 

 close relation with mesodermic vascular tissue lying close under 

 the brain, and in the latter case the subneural gland has similar 

 relations with the subneural sinus under the main ganglion. It 

 is even possible tliat the base of the '^ ueuropore^^ might be com- 

 pared with the infundibulum. As in the case of the Tunicata, 

 it is probable that the line from brain (infundibulum) to 

 hypophysis marks the posterior boundary of the pre-oral lobe. 



It is not so safe to attempt to draw the posterior boundary 

 of the collar-region. In both the Cephalochorda and 

 Vertebrata there can be no doubt that the trunk region is 

 hypertrophied, and also that the position of the anus has 

 changed, perhaps more than once. At the same time, in these 

 forms and in the Urochorda there are always traces of 

 hypoblast other than the notochord (post-anal gut, &c.), which 

 indicate that the prolongation of the notochord in a posterior 

 direction was part of the process of a movement of the whole 

 gut towards the posterior end of the body. There is no 

 difficulty in reconciling the relations of the notochord to gut 

 and blastopore in the embryos of Vertebrata, and the idea 

 of a primitive condition of collar notochord. In fact, the 

 elongation of the notochord posteriorly by a proliferation of 

 cells at the anterior end of the blastopore is readily interpreted 

 by this assumption. 



Again, a careful consideration of the structure of Actino- 

 trocha may make it possible to trace the chordate stock 

 practically to the Coelenterate stage, or at least to a rudi- 

 mentary triploblastic type. 



