No. 2.] THE HYPOPHYSIS OF AMI A CALVA. 71 



During this early period there is a distinct stratification in 

 the arrangement of cells in its lower portion, as if formed in 

 successive strata by proliferation from the mother layer. This 

 stratification is not apparent in the upper part of the organ, 

 and is no longer seen in the lower part after the ninth day. 



The first lumen is formed about the ninth day, from which 

 time on an increasing number of lumina is to be found as the 

 organ develops. A lumen appears near the end of each lobe. 

 These lumina are oval or spherical cavities, and definite chan- 

 nels of communication have not been observed, though indi- 

 cations that such channels are forming are found in the 

 arrangement of the cells about the longitudinal axes of the 

 lobes. The cells have a tendency to arrange themselves radi- 

 ally about the lumina. 



The organ is almost perfectly lens-shaped until about the 

 fifteenth day, when the formation of lobes begins with the 

 interpolation of mesoblastic cells between the hypophysis and 

 the brain. The number of lobes multiplies from this time on 

 till the thirty-fifth clay, beyond which stage observations have 

 not been made. The lobes form chiefly around the periphery 

 of the lenticular body. 



The organ is thus, at this stage, a spongy body with isolated 

 cavities, rather than a complex of glandular tubules which so 

 frequently characterize it. The nuclei of the cells become in- 

 distinct or wholly disappear by the twenty-second day. This 

 may indicate a glandular modification, but no evident glandular 

 secretions have been detected. No duct nor external opening 

 of cavities has been observed. 



No arteries or blood vessels are to be found in it at the 

 latest stage examined. Neither have nerve fibers been seen 

 connecting it with the brain. 



The cranial cartilages developing from the mesoblast increase 

 in size and strength from about the tenth day, until at the 

 thirty-fifth day they closely surround the organ on all sides, 

 forming the pituitary fossa, in which the organ lies in close 

 contact with the infundibulum on the dorsal side, but sepa- 

 rated from the mouth by a fibrous strand of connective tissue 

 on the ventral. 



