274 ZELENY. [VOL. II. 



a distinct limiting line between the epiblast and hypoblast at 

 the point where the hypophysis will later appear. 



After the mouth opening has appeared (Stage C, Fig. 4) we 

 find the hypophysis at first in the same relative position as in 

 Stage B. Then the great increase in size of the fore-brain 

 forces the epiblastic pocket to a position far back in the 

 pharyngeal cavity, so that Stages D and E when considered 

 alone would lead one to believe that the hypophysis is of 

 hypoblastic origin in this group. That such is not the case is 

 made evident by following the whole series of changes through 

 all the different stages. There can be no doubt that in the 

 Chelonia at least the oral evagination which goes to form the 

 hypophysis is of epiblastic origin. As regards the infundibular 

 portion there is no essential difference of opinion and its devel- 

 opment need not be touched on here. 



The bearing of the above conclusions on the paleostome 

 theory of Kupffer and the neostome theory of Dohrn is of 

 some interest. According to Kupffer, the hypophysis was 

 originally a canal connecting the fore-gut with the epiblast, 

 and represents an ancestral oesophagus which came up in 

 front of the fore-brain and was replaced by the modern oesoph- 

 agus at the time when the mouth was forced to a more ven- 

 tral position by the enlargement of the brain. Dohrn, on 

 the other hand, has picked out the epiphysis and hypophysis 

 as remnants of the old annelid oesophagus which went up 

 through the brain. Both of the above views presuppose some 

 connection of the hypophysis with the hypoblast. The sections 

 of turtle embryos which are described in the present paper 

 give no evidence of such a connection at any stage. 



In conclusion I wish to express my sincere thanks to 

 Prof. H. F. Nachtrieb, who suggested to me the investi- 

 gation of chelonian development and has aided me in many 

 ways during the progress of the work. 



DEPARTMENT OF ANIMAL BIOLOGY, 



UNIVERSITY OF MINNESOTA, December, 1900. 



