EBEN J. CAREY 



73 



The precocious segregation of the esophageal epithelial cells would 

 appear to be distinctly disadvantageous, for here a much larger pro- 

 portion of each cell is in contact with others than in the looser texture 

 of the mesenchyme. The intercellular vacuoles filled with fluid seen 

 in the epithehum of the esophagus of a 24 mm. pig embryo (Fig. 6) is 

 a provision for aiding excretion. These intercellular vacuoles are 

 comparable to the periodic appearance of intercellular cavities ob- 

 served by Kofoid in Limax. 



% 1-50 



i 



f:fS 



-I 



6 7 8 3 fO 11 fZ /i i^ /5 16 17 18 19 ZO 21 2Z Z3 Z9 

 leno7k ^/ ernlirya m Ml77i:>nefers 



Fig. 16. Curves of ratio of diameter to length of tlie epithelial tubes of the 

 esophagus and trachea. Note particularly that the esophageal tube grows 

 relatively more rapidly in width in embryos 10 to 15 mm. in length, whereas, in 

 embryos from 15 to 24 mm. in length the epithelial tube grows relatively more 

 rapidly in length. In the former period the inner, close spiral, muscle coat is 

 differentiating; toward the end of the latter the outer, elongated, spiral coat is 

 being formed. The relatively greater grow^th in width than in length of the 

 tracheal epithelial tube is objectively evident. This rapid growth is the efficient 

 cause in compressing the tracheal mesenchyme, acting as the stimulus in the 

 differentiation of the tracheal cartilage. 



