42 HAROLD SAXTON BURR 



example, a section through the hea/l of a young larva at the level 

 of the eyes, shows roughly three circles, the two optic vesicles 

 and the diencephalon enclosed in a fourth, the skin. Between 

 these organs above and below lie two triangular depressions filled 

 with mesenchyme. From the two ventral masses arise the tra- 

 becular As the eyes become separated from the brain the 

 mesenchyme filling the depression above shifts, gradually becom- 

 ing chondrified in the process and forming the crista. Exactly 

 the same procedure may be imagined to occur at the level of the 

 olfactory placodes. Here, however, owing to the greater extent 

 of the olfactory nerve, no crista is formed, the mesenchyme of 

 the upper depressions becoming secondarily chondrified to form 

 the ethmoid column. 



If now the placode of one side is removed all of the mesen- 

 chyme filling the depression simply and naturally joins the 

 trabecular mesenchyme. Hence, no ethmoid column is formed, 

 the mesenchyme from which it is normally developed becom- 

 ing united with that of the trabecula, greatly adding to its mass. 



Figures 5 and 8 from drawings of the model will show that the 

 crista too, of the operated side is not so high as its fellow. It is 

 quite evitient that some of the mesenchyme tissue destined to 

 form the cartilage of the crista has, owing to the absence of the 

 placode and olfactory nerve which usually serves as a dam to 

 keep it in position, simply slid forward, reducing the crista form- 

 ing mesenchyme and adding again to the trabeculae. 



The presence of the antoribital process on the operated side 

 indicates that the removal of the nasal sac has not eliminated 

 some factor that determines its formation, though the irregularity 

 in shape and position shows the lack of some formative influence. 



The presence of a rudimentary plate of cartilage in the general 

 position of the solum nasi seems to point to a formative influ- 

 ence still present in that region. A careful study of the early 

 stages shows that this plate does not develop until the bony 

 connection is established between the ascending process of the 

 premaxilla and the maxilla. This latter plate in all probability 

 molds into shape laterally a portion of the mesenchyme which 

 later chondrifies to form the rudimentary solum nasi. 



