64 DYNAMICS OF HISTOGENESIS. II 



is transformed into a muscle cell. To this end the writer is directing 

 his attention. It must not be forgotten, however, that valuable sug- 

 gestions pointing to a tensional stimulus as a factor in myogenesis is 

 derived from a study of the origin of this tisue in a closely graded and 

 advancing series of embryos. In the latter case direct observation 

 reveals what is actually going on in nature's own laboratory. 



Observations on the Early Development of the Esophagus of the Pig. 



Hitherto, descriptions of esophageal development were written 

 from a view-point regarding histogenesis as passive. No correlation 

 of the developing epithelial tube and the surrounding mesenchyme 

 has been presented. That one element could influence the other 

 during the critical genetic steps has been overlooked. In order to 

 comprehend clearly the development of the esophagus, or any forma- 

 tive structure for that matter, the active, dynamic point of view must 

 be possessed by the observer and not the purely passive one. The 

 interaction and interdependence of integers united in a common 

 structure must be considered together and not as isolated, non-related 

 entities. It is from the dynamic aspect then that the following ob- 

 servations differ from previous accounts of esophageal genesis. It is 

 also to be noted that certain developmental gaps are herein filled, 

 thereby making the sequence of histogenetic events objectively 

 evident. 



Corresponding portions of the middle of the esophagus from an 

 ascending series of embryos were selected for comparative study. 

 This region was primarily chosen due to the fact that the lower cer- 

 vical and upper thoracic portions have become narrowed due to the 

 elongation of the esophagus concomitant with the descent of the 

 stomach. The diminution in the diameter of this zone, leads to a 

 more rapid revolution of the epithelial mitotic figures on account of 

 the decrease in circumference of the epithelial tube. This more rapid 

 rate of rotation causes a corresponding greater vortical agitation in 

 the less active fluid like mesenchyme. The reacting maelstrom is 

 consequently more clearly seen and the path of the activating mitotic 

 figures more easily followed. The muscular rim bordering the mes- 

 enchymal vortex is also more clearly defined. 



