THE ORGANS OF THE INTERMEDIATE LAYER OR MESEXCHYME. 593 



39. The thoracic cavities have for a time the form of tubular spaces 

 lying on the dorsal side of the heart and on either side of the spinal 

 column ; they receive the developing lungs, and still communicate 

 caudad with the abdominal cavity. 



40. The two thoracic cavities are separated from the abdominal 

 cavity by the fusion of the dorsal rim of the septum transversum 

 with peritoneal folds of the dorsal wall of the trunk (the pillars of 

 USKOW). 



41. The diaphragm is composed of two parts, the ventral septum 

 transversum, and a dorsal part, the pillars. 



42. Upon its first establishment the liver grows into the septum 

 transversum, but subsequently detaches itself from the latter and 

 remains united to the diaphragm by means of its peritoneal covering 

 only, the coronal ligament. 



II. The Development of the Skeleton. 



With the exception of the chorda dorsalis, which takes its origin 

 from the inner germ-layer, the skeleton of Vertebrates is a product 

 of the intermediate layer, resulting from a series of histological 

 differentiations, a general survey of which has already (p. 540) been 

 given. There have appeared many articles treating on this very 

 complicated apparatus in the higher Vertebrates from a develop- 

 mental and also especially from a comparative-anatomical standpoint. 

 By an exhaustive treatment of this subject this part of the work 

 would attain to greater proportions than the plan of the present text- 

 book permits. I shall therefore limit myself to the more important 

 conditions of organisation and for what remains refer to the text- 

 books of comparative anatomy. 



Two chief parts are distinguishable in the skeleton of Vertebrates : 

 (1) the axial skeleton, which is in turn divisible into that of the 

 trunk and that of the head, and (2) the skeleton of the limbs. 

 The former is the older and more primitive, being possessed by all 

 Vertebrates ; the latter has been developed later, and is entirely 

 wanting in the lower groups (Amphioxus, Cyclostomes). 



A. The Development of the Axial Skeleton. 



The original foundation of the axial skeleton of all Vertebrates 

 is the notochord or chorda dorsalis. By this is understood a 

 flexible, rod-like structure, which is situated in the axis of the body 



38 



