130 THE THIRD DAY. [CHAP. 



At a point immediately behind (or when the embryo is placed 

 on its face, above) the heart, the cavity of the alimentary canal 

 is compressed laterally, and at the same time constricted in 

 the middle so that its transverse section (Fig. 43, 1) is some- 

 what hour-glass-shaped, and shews an upper or dorsal cham- 

 ber d, joining on to a lower or ventral chamber I by a short 

 narrow neck. Th& lower chamber then becomes broader 

 than high (Fig. 43, 2) while its under wall is raised up in a 

 median fold, which partially divides the chamber into two 

 lateral parts (Fig. 43, 3). As a result of these folds, the 

 original simple tube becomes divided into three grooves or 

 incomplete tubes, whose cavities all communicate with each 

 other at the centre of the original canal : into a single tube 

 above, which is the true oesophagus, and into two tubes 

 below, each of which is a rudiment of a lung. The presence 

 of these three incomplete tubes may be traced, by sections 

 at various levels, for a certain distance along the alimentary 

 canal, and are then lost, the canal once more returning to 

 the condition of a simple tube. 



The median fold or partition then rises up so as com- 

 pletely to shut off the three cavities from each other (Fig. 

 43, 4). The isolation commences behind and travels thence 

 forwards, but never quite reaches the point in front where 

 the division into three cavities begins. As a consequence 

 the lower pulmonary tubes, though closed behind, open into 

 the oesophagus in front. In other words, by the constriction 

 and median folds which we have described, the two short 

 pouches or diverticula of the lungs are developed from the 

 under (or anterior) surface of the oesophagus. 



At their first origin both diverticula together with the 

 alimentary canal itself are invested in one common rounded 

 mass of mesoblast whose external contour bears no marks of 

 the internal changes which are going on. By and bye, as 

 the diverticula diverge behind from the median line, they 

 carry the mesoblast with them (Fig. 43, 4), Then for the 

 first time they become evident from the outside. 



The subsequent history of these diverticula may with convenience be briefly 

 described here. 



At first the two diverticula have separate openings into the oesophagus ; but 

 as the point at which they enter is carried further forwards (or rather as they in 

 the process of growth are carried backwards), they unite near their base to form 



