443 



communication. This is limited to the observation of the organo- 

 genetic processes in the oesophagus about the level of the point of origin 

 of the two main bronchi from the trachea. A complete description of 

 the whole organ with a consideration of the literature will appear in 

 another place. 



In a pig embryo 3,5 mm nape breech measurement, the head gut, 

 in passing caudalwards from the last gill pouch, which has a flattened 

 rhomboidal appearance in cross section, narrows gradually until its 

 lumen forms an asymmetrical sagitaily placed fissure. Its epithelial 

 lining is quite variable in thickness, showing, in the dorsal angle, a 

 single layer of low columnar or cubical cells and, at the sides, columnar 

 cells two or more rows in thickness. The entire head gut, at this 

 stage, is surrounded by a mesoderm, composed of a syncitium of an- 

 astamosing cells in which the exoplasmic or fibrillar portion is not 

 well differentiated. At 4,5 mm, the longitudinal furrows indicating the 

 line of separation between the future trachea and oesophagus have 

 appeared and, in consequence, the head gut has, in cross section, more 

 or less of an hour glass shape and is, at the same time, more sym- 

 metrical with reference to the median longitudinal plane. As in the 

 younger stages, the epithelium is lower in the dorsal angle and 

 thicker at the sides. The character of the mesoderm, likewise, shows 

 no marked changes. 



At 5 mm, the separation of the trachea and oesophagus has been 

 inaugurated, and, in a pig 6 mm in length, it is well advanced, the 

 communication between the two segments persisting only at the upper 

 end. The primitive oesophagus is now oval in shape and in lined 

 throughout with a double layer of cells, the inner of which is distinctly 

 columnar in shape. The concomitant changes that take place in the 

 mesoderm are limited to a slightly greater difl'erentiation of the fibrillar 

 portion of the syncytium. 



In a pig 13 mm long (Fig. 1), stained with Mallory's method, 

 the oesophagus is oval in contour and shows a thickening of its epi- 

 thelial lining. Instead of the two or three layers of cells, there are 

 now four or five. Particularly is this true at the sides of the oeso- 

 phagus, for, as in the younger stages, the epithelium thins out at the 

 dorsal and ventral angles of the oval epithelial tube. The inner layer 

 of cells is more columnar, while the outer rows are smaller and more 

 polygonal in shape. The nuclei are round or oval and, here as in 

 most embryonic tissues, the mass of the nucleus is much greater in 

 comparison to the mass of cytoplasm, when the conditions in fully dif- 

 ferentiated tissues is taken as the standard. Without the outer layer 



