XXXVIII. 



THE COELOM AND THE MESENTERIES 



In our account of the earthworm, the student was introduced to all 

 higher forms of animals possessing a coelom or body-cavity. The chap- 

 ter on the earthworm should be reviewed at this point. 



Then,, too, in the early part of our work on chick embryology, we 

 have seen how the mesoderm divided into splanchnopleure or somato- 

 pleure and how the organs growing out from their respective beginnings 

 pushed a layer of one of these coverings before them. And we have also 

 seen how the chick embryo is quite similar to an animal which has had 

 a ventral incision made along the midline and then had these two halves 

 stretched over a yolk-sphere, so that its organs or portions of organs 

 which developed from two primordia or beginnings, could later come to- 

 gether when the fusion of the ventral body walls produced a single organ 

 of the two separated halves. 



In adult birds and mammals, the coelom or body-cavity consists of 

 three regions, known as pericardia!, pleural, and peritoneal. The pleural 

 region is paired, each half containing one lung. The other two chambers 

 are unpaired. The pericardial region contains the heart, and the peri- 

 toneal contains all the abdominal viscera. 



As the coelom arises by a splitting of the mesoderm, and the two 

 halves of the chick are spread out over the yolk, the coelom is naturally 

 a paired cavity, only becoming a single cavity when the ventral body 

 walls of the embryo come together, and the ventral mesentery then dis- 

 appears. 



There are no segmental pouches in the chick coelom as there are in 

 some of the lower vertebrates, though it cannot be said that this is unlike 

 the lower forms, for, by the time the coelom appears in the chick, the 

 pouches would already be broken through anyway, and have become 

 connected. 



As the mesoderm splits and the splanchnopleure and somatopleure 

 extend out over nearly the entire yolk-sac, it is to be understood that 

 much of this split mesoderm is extra-embryonic. This has already been 

 described in an earlier chapter. 



Here we are concerned with the embryonic coelom. 



The portion of the embryonic coelom which gives rise to the three, 

 body-cavities mentioned above is marked off by a series of folds which 

 separate the body of the embryo from the yolk. With the closure of 

 the ventral body walls, the embryonic coelom becomes completely sepa- 

 rated from the extra-embryonic, though in the yolk-stalk region it re- 

 mains open much longer than in other portions (Fig. 281, C to G). 



