Reproduction 



287 



Fig. 237. Diagrams showing the relations of the coelomic cavities (black) in 

 (A) fishes, (B) amphibians and Sauropsida, and (C) mammals. (D) Diaphragm: 

 (L) liver; (P) lungs; (S) septum transversum. In (B) the lungs lie in the peritoneal 

 (or pleuroperitoneal) cavity; in (C) they occupy special pleural subdivisions of the 

 coelom. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Phila- 

 delphia, The Blakiston Company.) 



pericardial cavity and continuous with the epicardium, are derived 

 from the adjacent hypomeric mesoderm. The heart muscle, however, 

 unlike that of blood-vessels, is striated. The coelomic space at either 

 side of the developing heart remains as the pericardial cavity. The 

 pericardium and epicardium correspond to the peritoneum of more 

 posterior regions of the coelom. 



The transverse septum, separating the pericardial from the ab- 

 dominal cavity, consists of pericardium in front and peritoneum be- 

 hind, with connective tissue between. The diaphragm of the mammal 

 is not the exact equivalent of the transverse septum of other verte- 

 brates (Fig. 237). That part of the coelomic space lying on the cephalic 

 side of the diaphragm is subdivided into three cavities, the pericardial 

 and the right and left pleural cavity containing the corresponding lobes 

 of the lungs. The diaphragm is muscular. Its muscle is striated and, 

 like body-wall muscle, is derived from epimeric mesoderm. Strangely, 

 however, it is mesoderm which shifts backward from somites of the 

 neck region. This accounts for the innervation of the diaphragm by 

 cervical spinal nerves. 



Head, Neck, Tail. The mesoderm of the head is less definitely 

 segmented than that of the trunk. There is nothing corresponding to 

 the mesomere of trunk mesoderm. The six muscles, consisting of stri- 

 ated fibers, which effect the movements of the eyeball in its orbit are 

 developed from head mesoderm which is probably the equivalent of 

 three somites or epimeres of the trunk. 



The neck region, whether or not differentiated externally, corre- 

 sponds approximately to that of the embryonic pharyngeal pouches. 

 In this region the dorsal mesoderm forms epimeres which give rise to 



