70 Vertebrate Embryology 



the notochord (Fig. 13, N). In the anterior 

 end of the embryo the mesoblast is much 

 thinner, and does not there split into the two 

 layers. 



On each side of the notochord the meso- 

 blast becomes thickened to form the segmental 

 plate, which does not, at first, show any sepa- 

 ration into two layers ; later, however, the 

 body-cavity does extend into the segmental 

 plate, for a time, but eventually disappears 

 from that region. 



At about the time when the medullary folds 

 are coming together to form the neural canal, 

 the segmental plate on each side of the noto- 

 chord begins to be broken up into blocks by 

 a series of vertical connective tissue septa 

 at right angles to the notochord. These 

 blocks or segments are the mesoblastic somites 

 or myotomes. The mesoblastic somites are, at 

 first, not separated from the lateral sheets of 

 mesoblast, but very soon after their formation 

 they become separated from the lateral meso- 

 blast, and, by the thickening of their walls, 

 especially the inner, their cavities are ob- 

 literated. At a somewhat later stage the 

 mesoblastic somites are largely converted into 

 muscles, whose >-shaped arrangement may be 



