76 THE FIRST DAT. [CHAP. III. 



(10) One or more pairs of mesoblastic somites make 

 their appearance in the vertebral portion of the meso- 

 blastic plates. 



(11) The first trace of the amnion appears in front 

 of the head-fold. 



(12) The vascular area begins to be distinguished 

 from the rest of the opaque area. 



