46 



TEXT-BOOK OF EMBRYOLOGY. 



ture around the ventral aspect of the gut until it meets its fellow of the oppo- 

 site side in the sagittal plane, thus separating ectoderm from entoderm. The 

 sagittal partition between the ccelomic spaces of the two sides then breaks 

 down and each side is in free communication with the other ventral to the 

 gut. The cells of the entire dilated structure have become decidedly flat- 

 tened except those in contact with the notocord and neural tube which 

 become more elongated columns and comprise the muscle plate or myotome 

 (Fig. 25). The portion of the cavity contiguous to the myotome is now 

 known as the myocoel while the remainder of the coelomic space is the splanch- 

 nocoel. Subsequently a partition appearing between the myoccel and 

 splanchnocoel completely separates the two cavities. The myotomes, in the 

 sites of the original somites, retain their segmental character. The parti- 



Neural tube 



Epidermis (ectoderm) 



Coelom 



Primitive segment 

 Intestine 



Entoderm 



Notochord 



Primitive segment 

 Muscle plate 

 Cutis plate 

 Myocoel 



Coelom 



Splanchnocc 

 Parietal mesoderm 

 Visceral mesoderm 



\ lat. plate 



Ventral Subintestinal 

 mesentery vein 



FIG. 25. Diagram to show differentiation of primitive segment into muscle plate (myotome) and 

 cutis plate and relation of myocoel and splanchnocoel. Bonnet, Compare with Fig. 22, d. 



tions between adjacent splanchnoccelic cavities, on the other hand, break 

 down and the common cavity thus produced, which is now known as the 

 coelom, no longer bears the segmental character but is continuous on both 

 sides of and below the gut. 



The biological significance of ectoderm and entoderm has been briefly 

 noted. Between these two layers the mesoderm appears and presently 

 begins to elaborate and to contribute to their support ; support in the broadest 

 sense of the term. As the organism continues to develop, the middle germ 

 layer becomes a framework within and around which the refinements of the 

 two primary layers are suspended. The whole series of connective tissues 

 is of mesodermal origin, and this applies even to the cartilaginous and bony 

 skeleton. The muscles, all three varieties, whose activities are associated 

 with motion and locomotion are derivatives of the mesoderm. The blood 



