THE PARS BUCCALIS OF THE HYPOPHYSIS 81 



a time the same thinning as do the dorsal and lateral walls. 

 By a 9.5-mm. stage the pituitary wall has become much thinned, 

 and consequently the various portions of the infundibular process 

 are approximately of the same thickness (fig. 10) and nearly 

 structurally identical. From this point a series of changes take 

 place leading to a very different condition in the pituitary wall 

 of the two types. In the albino the membranous wall persists. 

 In the normal this becomes progressively thicker and finally 

 attains a thickness many times that obtaining in the 9.5-mm. 

 stage. It is significant that the epithelial hypophysis, although 

 previously in contact with a small portion of the infundibular 

 floor prior to a 10-mm. stage, institutes an intimate juxta- 

 position with the pituitary wall of the infundibular process at 

 the time when the latter loses its membranous character and 

 begins to thicken. 



Differences between the neural lobe of the albino and the 

 normal appear at a later stage than they do in the pituitary 

 wall. The thickening near the apex of the infundibular process 

 indicating the first formation of the neural lobe makes its appear- 

 ance at approximately a 14-mm. stage in both the normal and 

 albinous tadpole, 44 and is nearly identical in position and size 

 in both types at a 16- to 17-mm. stage. From this stage the 

 neural lobe in the normal increases concomitantly with the 

 growth and lateral extensions of the developing pars intermedia. 

 In the albino, on the contrary, a slower growth (table 6) and 

 relatively slight lateral extension take place a process leading 

 to the small, malformed neural lobe of the albino. 



The small neural lobe and the membranous pituitary wall 

 of the albino are thus clearly referable to a non-development 

 rather than to an atrophy of a normally sized structure. The 

 walls of the early infundibular process have apparently two 

 inherent possibilities after having undergone the primary thin- 

 ning : one,, to retain this membranous characteristic as do both 

 the dorsal and lateral walls of the normal or albinous larvae 

 and the pituitary wall of the albino; the other, to develop into 



44 According to Atwell, this lobe becomes recognizable in an 18 to 20 mm. stage 

 in R. pipiens. 



MEMOm NO. 11. 



