REMOVAL OF NASAL PITS AMBLYSTOMA EMBRYOS 43 



The structure of the normal nasal cartilages shows quite 

 clearly that they are an almost perfect ' cast' of the underlying 

 nervous tissue. The removal of the 'core,' the nasal sac, of 

 the capsular cartilages causes a collapse of the whole structure 

 with the subsequent settling of the cartilage forming material 

 to the lowest point. 



It may be concluded from these experiments that there exists 

 in the head a certain amount of mesenchyme tissue destined to 

 form cartilage. The general shape of these cartilaginous struc- 

 tures is determined by the form of the underlying organs. The 

 removal of any such underlying parts results not in any reduc- 

 tion in the mass of cartilage formed, but in the rearrangement 

 of the cartilage forming tissue to meet the change in the form 

 of the 'core' about which it is formed. 



II. Development of naso-lacrymal duct 



In addition to the modifications of the capsular structures, it 

 was noted that the naso-lacrymal duct present in the 33 mm. 

 larva, on the unoperated side was entirely absent on the operated. 



At present the most widely held theory of the formation of 

 the naso-lacrymal duct is that it is a solid cord of cells growing 

 centripitallj^ toward the nasal epithelium from the point of origin 

 in the ectoderm on the dorso-anterior aspect of the optic anlage. 

 This solid cord becomes secondarily canalized. 



Figures 10 and 11 {nl) show respectively the junction of the 

 cord with the nasal epithelium and the surface ectoderm. It is 

 rather thicker at the point of union with the nasal sac than where 

 it meets the ectoderm and seems to run more gradually into the 

 sac than it does into the ectoderm. There is also a marked con- 

 tinuation of the cavity of the sac into the cord indicating that 

 the canalization occurs from this end. Born in 1876 worked 

 out the development of this duct. He found that it first 

 appears as a solid cord of cells reaching from the surface ecto- 

 derm to the nasal sac and that it is derived from the former. 

 The work of Schaeffer in 1912 on man has shown that the duct 

 is at first a solid cord derived from the inner layer of the epidermis. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 20, NO. 2 



