No. 2.] THE HYPOPHYSIS OF AMI A CALl.l. 63 



brain remains one cell in thickness and rests flatly upon the 

 dorsal wall of the gut and the hypophysis. The sucking disc 

 (sc.} is relatively at its largest size. The cavity between it and 

 the brain is filled with a denser aggregation of mesoblastic cells 

 than in the previous stage. 



Fig. 5 shows the base of the brain and the hypophysis of the 

 same section more highly magnified. The hypophysis now 

 measures i 50 yu, in length and 28 ^ in thickness. This apparent 

 shortening may be due to the uncertainty of the limits of the 

 organ in the previous stage, or to individual variations in the 

 two larvae. That peculiar nesting of the cells in the hypophysis, 

 as described above, will be seen to continue, but the cells are 

 now perceptibly larger, while the epithelial cells (em.} forming 

 the roof of the mouth beneath the hypophysis have again 

 attained the size and shape of those with which they are con- 

 tinuous in front and behind. The basal layers of endoderm, 

 however, still seem to be dividing, adding new layers to the 

 base of the hypophysis by proliferation. The roof of the 

 mouth is now three layers deep both before and behind the 

 hypophysis. 



A transverse section (Fig. 6) of a larva about eight days old, 

 through the posterior part of the hypophysis and the infun- 

 dibulum, shows that the organ is at this stage convex on the 

 dorsal side in a transverse plane, fitting into a concavity in the 

 base of the infundibulum lying closely upon it. An examina- 

 tion of the successive sections forwards and backwards from 

 this shows that the upward convexity gradually diminishes 

 forwards, but more abruptly backwards. This figure, in con- 

 nection with the figures of longitudinal sections of larvae both 

 older and younger, shows that the organ is now approximately 

 lenticular in shape. In transverse, as in longitudinal sections, 

 that characteristic nesting of the cells differentiated to form 

 the hypophysis is found to prevail, a basin-shaped stratum of 

 lenticular cells formed from the deeper layers of the endoderm, 

 with other similar strata, each successively smaller, fitting into 

 the previous ones, until we get a nest of seven or eight basins, 

 while the hollow of the upper basin is filled to a rounded full- 

 ness with cells of a more nearly uniform diameter, rounded or 



