70 P RATHER. [VOL. I. 



SUMMARY. 



In distinction from an epiblastic origin, as found in most 

 forms, my observations lead me to believe that the hypophysis 

 is of hypoblastic origin in Amia. 



Prior to the differentiation of the hypophysis the foregut 

 extends far forwards ; by diverticula the hypoblast reaches 

 even the dorsal side of the head in front of the brain. At 

 this time the stomodaeal involution is below the front end of 

 the foregut. The diverticula later sever their connection with 

 the foregut and are metamorphosed into the larval adhesive 

 organ. The hypoblast unites with the epiblast on the last day 

 before hatching, seventh day, forming the oral plate at a point 

 forwards of the anterior limit of the brain. 



There is no indication of an overgrowth of epiblast between 

 the brain and the foregut, nor is there an invagination from 

 the stomodaeum to give rise to the pituitary body. Neither 

 does an outfold from the hypoblast occur to form it. Its first 

 stage is found near the close of the embryonic period, about 

 1 60 hours, as a local differentiation of hypoblastic cells in the 

 dorsal wall of the mesenteron, immediately under the thala- 

 mencephalon where the base of the brain is in close contact 

 with the hypoblast. The intimate fusion of the base of the 

 first primary vesicle with the hypoblast, from a time long 

 before the stomodaeum has united with the foregut until after 

 the hypophysis has become well differentiated, seems to me 

 to preclude the possibility of any epiblastic tissue entering 

 into its composition, and leads me to think that its origin is 

 probably due to a mechanical cause. This region of fusion is 

 far back of the oral plate, which remains intact for several 

 hours after the differentiation of the hypophysis has begun. 



The growth of the hypophysis is at first apparently due 

 more to the enlargement of the cells first differentiated to 

 form it than to the addition or multiplication of cells. It 

 appears to remain in genetic connection with the mother layer 

 until about the tenth clay, when it becomes wholly delimited 

 therefrom by an ingrowth of mesoblastic tissue between it and 

 the mouth roof. 



