REMOVAL OF NASAL PITS AMBLYSTOMA EMBRYOS 41 



the upper, so that where the upper jaw curves in as a result of 

 the operation, the lower jaw follows. This brings the teeth into 

 apposition at all times. The maxillary teeth are unaffected by 

 the operation. They are normal as to both size and number on 

 the operated side. 



The process by which the above changes in structure are 

 formed is relatively simple. In the operated larvae, the tra- 

 becula of the operated side appears simultaneously with that of 

 the unoperated side and for some little time no difference can 

 be observed in the development of the two sides. 



Soon after the larva has begun to feed, it becomes evident that 

 the trabecula of the operated side begins to lag behind its fellow 

 in the advancing chondrification. At the same time that this 

 observable retardation in longitudinal growth occurs there is a 

 corresponding increase in the cross section of the trabecula. 

 The antorbital process appears at this time though it is somewhat 

 farther forward that its normal fellow. As development pro- 

 ceeds it becomes more and more atypical, finally appearing as 

 an irregular projection from the trabecula at the level of the 

 posterior terminus of "the maxilla. 



Coincident with thte growth of the cartilages in th# region of 

 the posterior nares, there appears laterally the plate-like chondri- 

 fications which eventually form the tectum nasi and the solum 

 nasi. The appearance of these on the unoperated side presages 

 a marked thickening of the trabecula of the operated side. The 

 end result of this is shown in figure 11 (t). Eventually the dis- 

 crepancy in the length of the trabecula mentioned above is com- 

 pensated for, both reaching forward to the premaxilla. 



It is evident then that correlated with the absence of the nasal 

 sac, there is an almost complete absence of the normal structures. 

 The process of this reduction may be largely explained on a 

 mechanical basis. 



In the early development of the head region, the brain, to- 

 gether with the anlagen of eye and nose fill practically the whole 

 of the anterior part of the head. There are, however, certain 

 inequalities in the contour of the organs which are filled with 

 mesenchyme, thus giving the smooth oval of the head. For 



