202 Journal of Comparative Xeurology and Psychology. 



of the head in Amblystoma. The relations described are fuUy 

 maintained until after the optic vesicle and stock are well 

 formed: indeed, the in\-agination of the h\-poph3-sis does not be- 

 gin until this time. It is therefore easy to determine that the 

 anterior end of the brain lies in front of the optic groove, that 

 is, just in front of the praeoptic recess of the adult. Immediately 

 in front of this is formed the anterior commissure, and above 

 this is the lobus olfactorius impar which marks the dorsal border 

 of the neuropore. 



It may be necessarj- to add that when I say just in front of 

 the optic groove I mean to indicate an appreciable distance, a 

 sufficient distance to have morphological value. If I were to at- 

 tempt to fix ven.' accurately the point in question in Amblystoma 

 I should put it nearer the anterior commissure than to tlie prae- 

 optic recess. Thus it will be seen that while my result is 

 closeh" related to that of His, there is a distinct difference which, 

 as later pages will show, is of considerable importance for seg- 

 mentation. It should be said, however, that the data on which 

 His based his conclusion would not exclude the result at which 

 I have arrived in the study of amphibia. 



Before accepting this result as final, it is necessary to com- 

 pare lower and higher forms. It seems certain that in the mat- 

 ter of the cephalic extent of the entoderm and mesoderm the 

 selachians are the most primitive vertebrates. In the mat- 

 ter of the branchial apparatus and hypophysis, however, the 

 cyclostomes present more primitive conditions. These condi- 

 tion? are ver\- well known from the work of Dohrn" (25), 

 KuPFFER 178) and Koltzoff iyS). The hj-pophysis reaches a 

 much greater developement than in other vertebrates and in 

 Bdellostoma (Kupffer) communicates openly with the archen- 

 teron before the definitive mouth opening is formed. In the 

 front wall of the hypophysial pit are formed at first a median 

 and afterward paired lateral ectodermal thickenings which later 

 form the nasal epithelium. The h}-poph\-sis fuses with the 

 foremost end of the entoderm and this comes into contact with 

 the brain at the infundibular region (Koltzoffj. Here we have 

 relations similar to those in selachians but with important differ- 



