THE KESPIKATOEY SYSTEM. 259 



lung buds. They are developed in mammals only, and are 

 evidently comparatively late structural additions. In the - for- 

 mation of these parts of the diaphragm, which close up the 

 pleuro-peritoneal communications, probably several other factors 

 take part. Of these may be mentioned : 



(1) The Wolffian ridges, which extend forwards as far as the 

 cervical region, and project from the roof of the coelom at the 

 passage between the peritoneum and pleura. 



(2) The formation of the supra-renal body (Minot). It is 

 developed in the mesoblast round this opening — -partly in the 

 septum, partly in the Wolffian ridge. 



(3) The growth of the liver within the septum transversum 

 probably plays an important part. Hence, probably, the greater 

 frequency of diaphragmatic hernia on the left side, where the 

 liver growth is less. By these various factors the openings are 

 closed and the diaphragm completed by the formation of the 

 region of the arcuate ligaments. 



The diaphragm begins to develop in the neck under the 4th 

 and 5th cervical segments. Processes from the muscle plates of 

 these two segments enter the basis of the diaphragm and form 

 part at least of its muscular substance. The muscle buds carry 

 their nerve — the phrenic — with them. The descent of the 

 heart, the retrogression of the stomach, and the development of 

 the lungs lead to the diaphragm being pushed backwards until 

 it assumes the adult position. 



Development of the Supra-renal Bodies. — In spite of much 

 research there is still doubt as to the origin and nature of these 

 bodies. The cortex and medulla are certainly of different origin. 

 In Elasmobranchs (sharks, etc.) these two elements are separate, 

 the cortical part forming an inter-renal, unpaired body; the 

 medullary part a body situated above the Wolffian kidney (supra- 

 renal) on each side, and closely connected with a ganglion of the 

 sympathetic system, in connection with which they are developed. 

 In vertebrates above fishes the cortical and medullary parts are 

 combined in one body. The medulla arises from the groups of 

 cells which form the sympathetic ganglia; probably from the 

 primitive cell basis of the semilunar ganglion, which is developed 

 in the septum transversum, close to the pleuro-peritoneal canal. 

 Hence the great plexus of nerves which pass from the solar 



