No. 6.] THE HYPOPHYSIS IN CHELONIA. 273 



back of the hypophysial evagination, so that the latter finally 

 appears to lie far back in the pharyngeal cavity. However, 

 here, as before, the dotted line //' shows the original position 

 of the now broken membrane. At this stage the notochord 

 has severed its connection with the fore-gut, but it is still 

 joined to the mass of cells which connects the two head cavi- 

 ties. These cells still surround a canal, so that there is a 

 passageway from the premandibular cavity on one side of the 

 head to the corresponding cavity on the other side. 



Stage F. 



Fig. 7, PI. IV, represents a median sagittal section of 

 Aspidonectes spinifer, and Figs. 8 and 9, PI. V, represent 

 sections of the same series, respectively, three and six sections 

 to the side of the median line. In these it is seen that there 

 is no evidence of the canal which connected the two head 

 cavities, and the notochord shows no sign of the former union 

 with the hypoblast. The hypophysis has begun to constrict in 

 the basal region and to enlarge in the terminal region so as to 

 show a division into a narrower basal stalk and a wider terminal 

 sac-like portion. 



At a considerably later stage than the above the infun- 

 dibulum sends out a pouch-like evagination, which from the 

 beginning is in close contact with the wall of the oral sac 

 and forms the infundibular part of the hypophysis. 



Summary and Conclusion. 



The foregoing series of sections furnish a clear chain of 

 evidence in favor of the epiblastic origin of the hypophysis 

 in Chelonia. Stage B (Figs. 2 and 3) itself is a conclusive 

 proof of such an origin. Here with the pharyngeal membrane 

 yet unbroken we find that the evagination is on the epiblastic 

 side of the membrane. The distinct limiting line which marks 

 the inner border of the hypophysial pouch excludes the suppo- 

 sition that the hypoblast cells may take a part in its forma- 

 tion. Even at the early stage represented in Fig. i there is 



