MYOTOMES AND CCELOM 



57 



Each primitive segment is thus differentiated into myotome and sderotome, while 

 its ventral part, concerned in the formation of the excretory ducts, may be termed 

 the nephrotome. The myotomes retain their segmental disposition, but the sclero- 

 tomes have really no separate identity, being at once fused into a continuous axial 

 sheet of mesenchyme. The primitive segments are not, however, the only source 

 of the embryonic connective tissue. The parietal and visceral plates of 

 mesoderm become likewise resolved into the mesenchyme of the body-wall and 

 gut- wall respectively, with the exception of the cells lining the ccelom, which become 

 the endothelial lining of the body- cavities (mesothdium). 



my 



w.d. 



FIG. 82. TRANSVERSE SECTION THROUGH THE TRUNK AND HIND-LIMB BUDS OP A RABBIT EMBRYO OF 

 THE TENTH DAY. THE LINE OF SECTION IS SLIGHTLY OBLIQUE. (T. H. BtyCC.) 



n.c., neural canal ; ???.., mesodermic segment ; sc, ventral wall of segment becoming resolved to form 

 sclerotome ; /////, outer wall of segment ; w.d., w.rf.,-Wolffian ducts ; on the inner aspect of each a cord of 

 nephrogenetic tissue; A 1 , A^, the two primitive aortse cut close to where they separate into the allantoic 

 arteries, A i , A 2 ; C, coelom ; G, gut. 



We have already seen that the mesoderm remains unsegmented in the region 

 of the head. Moreover, there is here no distinction between paraxial and lateral 

 mesoderm, and no splitting to form coelomic spaces. The whole unsegmented 

 mesoderm becomes resolved into a continuous mesenchyme which surrounds the 

 cerebral vesicles and the head end of the notochord. 



In lower vertebrates certain ccelomic cavities appear in the head which are 

 concerned in the formation of the muscles of the eyes, and of the branchial region. 

 They correspond to the preoccipital head-segments already alluded -to, and are 



