674 



MANUAL OF PHYSIOLOGY. 



tudinal partition, so that on either side of the neural canal is a 

 mass of undivided mesoblast extending laterally towards the 

 divided mesoblast. 



In each vertebral plate there appear transverse vertical inter- 

 ruptions at definite intervals which split the plate up into a num- 



FIG. 264. 



FIG. 203. embryo chick at the end of the second day, seen from below. 

 (Kolliker.) FA. Forebrain. Ab. Optic vesicles. Oh. Notochord. If. Heart. 

 om. Omphalo-mesenteric veins. Vd. Lower opening of foregut. 



FIG. 264. Division of the vertebral column of a chick. (Kolliker after 

 Remak.) 1. Notochord. 2. Points of separation of the original protover- 

 tebrse. 3. Points of division of the permanent vertebrae. 4. Arches of the 

 vertebrae. 5. Spinal ganglia, c. Body of first cervical vertebra, d. One 

 of the lower vertebra?. 



ber of quadrangular blocks of mesoblast, known as the protover- 

 tebrce ; the number of these corresponds to the number of vertebrae 

 of the animal. 



These protovertebrse become subdivided by transverse fissures 



