70 THE FIRST DAT. [CHAP. 



are due to breaches of continuity in the mesoblast 

 only, the epiblast and hypoblast having no share in the 

 matter. 



Thus each vertebral plate appears in surface views 

 to be cut up into a series of square plots, bounded by 

 transparent lines (Fig. 23). Each square plot is the 

 surface of a corresponding cubical mass (Fig. 24, Pv.). 

 The two such cubical masses first formed, lying one on 

 each side of the notochord, beneath and a little to 

 the outside of the medullary folds, are the first pair 

 of mesoblastic somites'. 



The mesoblastic somites form the basis out of which 

 the voluntary muscles of the trunk and the bodies of 

 the vertebrae are formed. 



The first somite rises close to the anterior ex- 

 tremity of the primitive streak, but the next is stated 

 to arise in front of this, so that the first-formed so- 

 mite corresponds to the second permanent vertebra. 

 The region of the embryo in front of the second formed 

 somite — at first the largest part of the whole embryo — is 

 the cephalic region (Fig. 23). The somites following the 

 second are formed in regular succession from before 

 backwards, out of the unsegmented mesoblast of the 

 posterior end of the embryo, which rapidly grows in 

 length to supply the necessary material. With the 

 growth of the embryo the primitive streak is con- 

 tinually carried back, the lengthening of the embryo 

 always taking place between the front end of the 

 primitive streak and the last somite ; and during this 



1 These bodies are frequently called protovertehrce, but we shaU 

 employ for them the term mesoblastic somites. 



