270 ZELENY. [VOL. II. 



true preoral gut and not an apparent one caused by the down- 

 ward bending of the head region ; for we must consider the 

 sharp angle in the hypoblast at Pr.G. in the figure, and not 

 the part back of it where the mouth will appear, to have been 

 the original extreme anterior end. Thus we may consider all 

 the hypoblast below and posterior to this angle to belong to 

 the ventral wall of the alimentary canal, and all that above and 

 posterior to belong to the dorsal wall. Following the dorsal 

 wall we find that it immediately divides into two parts, one of 

 which is the notochord (Nc.) and the other the dorsal wall 

 of the gut (End. 2). For this reason the hypoblast forming the 

 ventral wall of the alimentary canal, and continued out at 

 the ends of the embryo into the flat outlying blastodermic 

 region, may be called the " primary hypoblast," and the hypo- 

 blast of the dorsal wall the " secondary hypoblast." Rex 

 ('97), in his work on the duck, and Davidoff ('99), in the 

 embryo of Platydactylus, have found similar relations of the 

 notochord to the hypoblast. The terms " primary hypoblast ' 

 and " secondary hypoblast," which are used above, are taken 

 from the paper of the latter author. It is important to note in 

 this connection that my sections show a distinct line of demar- 

 cation between the epiblast and the hypoblast in this region, so 

 that the mass of cells surrounding the preoral gut is distinctly 

 hypoblastic and not a mass of undifferentiated cells. The 

 arrow with a feathered shaft (Y) shown in Fig. i in the 

 epiblast directly under this point marks the position and direc- 

 tion of the future hypophysial pocket. The very plain line of 

 division between the epiblast and hypoblast excludes the possi- 

 bility that any of the hypoblast cells may take part in this 

 ingrowth of the epiblast. 



Stage B. 



In an embryo 5.2 mm. in length and with twenty-one meso- 

 blastic somites, such as is shown in median sagittal section in 

 PL I, Fig. 2, and PI. II, Fig. 3, the brain has developed with 

 great rapidity. The cephalic flexure having proceeded at the 

 same time, the region around the preoral gut is greatly com- 

 pressed. The cavity of the preoral gut itself has become very 



