DEVELOPMENT OF THE ALIMENTARY TRACT. 



1 7c I 



cavity, and extends from the ventral wall to the sinus venosus, its median part be- 

 neath the heart being attached dorsally to the gut-tube, while its lateral expansions 

 form the floor of the pleural coelom. This imperfect partition, the septum trans- 

 versiim of His, also affords passage for the two ducts of Cuvier, formed on each side 

 by the union of the primitive jugular and cardinal veins, to gain the sinus venosus ; 

 the septum transversum receives the hepatic outgrowth from the primitive duodenum, 

 which soon develops a conspicuous liver-mass within the substance of the septum. 

 The rapid increase in the mass of the developing liver is attended by great thicken- 

 ing of the septum transversum, particularly towards its dorsal edge. Coincidently 

 with this augmentation, the septum differentiates into a thinner upper and a thicker 

 lower stratum, the former constituting the floor of the pericardial cavity and sur- 

 rounding the ducts of Cuvier, the latter enclosing the liver. 



Fig. 1436. 



Trache? 



Pericardial sac 



Septum transversum 



Loop of small 

 intestine extend- 

 ing into cord 



Vitelline vessels' 



Caecum 



(Esophagus 



Lung 



Communication 

 between pleural 

 and peritoneal sacs 



i Suprarenal body 



Aorta 



Wolffian body 

 Mesogastrium 



Duodenum 

 Kidney 



Wolffian duct 

 Ureter 



Reconstruction of human embryo of 17 mm. vertex-breech length. X 14. (Mall.) 



The subsequent development of the liver is attended by progressive, although 

 only partial, separation of the inferior layer from the superior stratum of the septum 

 transversum, the latter layer remaining as the primitive, but still imperfect, dia- 

 phragm between the pleuro-pericardial and peritoneal divisions of the body-cavity. 

 The dorsal attachment of the septum trans\'ersum, at first high in the cervical region, 

 gradually recedes tailward. On reaching the level of the fourth cervical segment the 

 fourth myotome is prolonged into the upper layer of the septum to supply muscular 

 tissue to what now becomes the diaphragm. The latter, however, is still incomplete 

 dorsally, owing to the existence on each side of the communication between the pul- 

 monary and peritoneal sacs. This opening is gradually closed by the backward 

 growth of the diaphragm and the forward and downward extension of the pleuro- 

 peritoneal membrane until the aperture between the thoracic and abdominal cavities 

 is effaced and the diaphragm is complete. 



