EBEN J. CAREY 65 



The characteristic embryological facts will be presented in a closely 

 graded series of pig embryos ranging from 9.5 to 24 mm. in length. 

 It is during this short period that the genesis and growth of the inner, 

 close spiral, smooth muscular coat is taking place. During the latter 

 part of this period the outer elongated spiral or longitudinal muscular 

 coat is becoming faintly indicated. 



In a 9.5 mm. embryo (Fig. 1) the esophagus possesses a small epi- 

 thelial tube composed of two to four rows of nuclei surrounding an 

 oval or elliptical lumen. Enclosing the epithelial tube are found 

 the undifferentiated mesenchymal cells. These are uniformly dis- 

 tributed in a spiral manner from the center to the periphery. The 

 vortical arrangement of the mesenchyme together with growth of the 

 esophagus in width and length depends upon the growth of the 

 epithelial tube (Fig. 15). 



The characters of the esophagus of pig embryos correspond closely 

 to those found by F. T. Lewis and Johnson for human embryos of 

 8.4 to 16 mm. in length. Near the larynx it is crescentic in shape 

 with the concavity of the crescent directed toward the trachea. This 

 is a compression concavity due to tracheal growth. Caudad, the 

 trachea first becomes round then transversely elliptical. In the upper 

 thoracic region near the bifurcation of the trachea it is rounded but 

 further caudad it merges with the stomach possessing a shape dorso- 

 ventrally elliptical. 



The lumen is pervious and contains an albuminous liquid demon- 

 strable in the living embryo. This liquid fonns a reticular coagulum 

 in fixed specimens (Figs. 1 to 6). The hydrodynamic influence of 

 the liquid content found throughout the alimentary canal during mam- 

 malian development has been entirely neglected. In older embryos 

 the accumulation of this intercellular fluid in isolated spaces which 

 subsequently become confluent with the lumen is a mechanical aid in 

 the separation and active moving apart of the cells resulting in en- 

 largement of the lumen. 



With further development the mesenchyme surrounding the esoph- 

 ageal tube becomes more discrete and defined from that enclosing 

 the trachea. This is clearly seen in a 11.5 mm. embryo as shown in 

 Fig. 2. The vortical arrangement of the cells is becoming more 

 clearly marked. The mesenchymal cells show varying degrees of 



