858 THE DEVELOPMENT OF THE COELOMIC CAVITIES 



cavities surrounding the lungs, and the peritoneal cavity in which He the 

 stomach, intestines, reproductive organs, etc. These coelomic spaces and 

 recesses arise from a generaUzed basic condition known as the primitive 

 splanchnocoelic coelom. The primitive splanchnocoeUc coelom is the elon- 

 gated cavity which extends throughout the trunk region beginning just anterior 

 to the heart and continuing posteriorly to the base of the tail. It encloses the 

 developing heart and the developing mesenteron (gut) from the esophageal 

 region posteriorly to the anal region. 



2. Origin of the Primitive Splanchnocoelic Coelom 



As observed previously (Chapter 10) the elongated mesodermal masses 

 lying along either side of the developing neural tube, notochord, and enteric 

 tube have a tendency to hollow out to form a cavity within. That is, like the 

 neural, gut, and epidermal areas of the late gastrula, the two mesodermal 

 masses tend to assume the form of tubes. 



In the case of Amphioxus, each individual somite forms a cavity, the 

 myocoel. These myocoels merge on either side in their ventral halves to form 

 an elongated splanchnocoel below the horizontal septum (see page 506). Later 

 the two splanchnocoels fuse below the developing gut to form the single 

 splanchnocoelic coelom which comes to surround the gut. In the vertebrate 

 group, however, the two elongated splanchnocoels on either side of the de- 

 veloping gut tube and heart form directly in the hypomeric (lateral plate) 

 area of the mesodermal masses without a process of secondary fusion as in 

 Amphioxus. In the upper part of each mesodermal mass, that is in the 

 epimere, and to some extent also in the mesomere (nephrotomic plate) in the 

 vertebrate group as in Amphioxus, there is a tendency for the coelomic spaces 

 to appear in segmental fashion within the primitive somites and within the 

 anterior portion of the mesomere. These individual spaces within the somites 

 are called myocoels, and the spaces which arise in the segmented portion of 

 the nephrotome are called the nephrocoels. 



In young shark embryos, such as the 3-4 mm. embryo of Squalus acan- 

 thias, and in amphibian embryos of the early post-gastrular period, the myo- 

 coelic and nephrocoelic portions of the coelom are continuous dorso-ventrally 

 with the splanchnocoelic coelom (fig. 217G and H). (Actually, during the 

 early stages of coelomic development within the mesodermal masses, in the 

 shark and amphibian embryos, the coelom within the epimere and nephro- 

 tomic portions of the mesoderm is continuous antero-posteriorly and it is 

 only after the appearance of the primitive somites and segmentation within 

 the nephrotome that they become discontinuous.) On the other hand, in the 

 embryos of higher vertebrates, the respective myocoels within the somites 

 appear later in development, and in consequence they are always separated 

 from the splanchnocoel. Similarly, the nephrocoelic coelom also arises later 

 and only the separate nephrocoels which develop within the pronephric tubules 



