150 EARLY EMBRYOLOGY OF THE CHICK 



of incubation the functional activity of the mesonephros is at 

 its height. After the eleventh day the developing metanephros 

 begins to become active and the mesonephros degenerates. 

 The establishment of the metanephros and the development 

 of the genital organs occur in stages which are too advanced 

 to come within the scope of this book. 



VII. The Ccelom and Mesenteries 



In adult birds and mammals the body cavity consists of three 

 regions, pericardial, pleural and peritoneal. The pleural divi- 

 sion is paired, each of the pleural chambers being a laterally 

 situated, sac containing one of the lungs. The pericardial 

 chamber containing the heart, and the peritoneal chamber con- 

 taining the viscera, other than the lungs and heart, are un- 

 paired. These regions of the adult body cavity are formed by 

 the partitioning off of the primary body cavity or ccelom of 

 the embryo. 



In the chick the ccelom arises by a splitting of the lateral 

 mesoderm of either side of the body (Fig. 54, A, B). It is 

 therefore, primarily a paired cavity. Unlike the ccelom of 

 some of the more primitive vertebrates, the ccelom of the chick 

 never shows any indications of segmental pouches correspond- 

 ing in arrangement with the somites. The right and left 

 coelomic chambers extend antero-posteriorly without interrup- 

 tion through the entire lateral plates of mesoderm. This dif- 

 ference in the formation of the ccelom does not imply any lack 

 of homology between the coelom of the chick and that of more 

 primitive forms. The process of coelom formation in the chick 

 may be considered as being accelerated with a resultant slurr- 

 ing over of the early phases. The coelom first appears in a 

 condition which is comparable with the coelom of m"ore primi- 

 tive forms at that period of differentiation when the segmen- 

 tally arranged coelomic pouches have broken through into each 

 other and their cavities have become confluent. 



The coelomic chambers are not limited to the region in which 

 the body of the embryo is developing. They extend on either 

 side into the mesoderm, which in common with the other germ 

 layers, spreads out over the yolk surface. A large part of the 

 primitive coelomic chambers thus comes to be extra-embryonic 



