672 



MANUAL OF PHYSIOLOGY. 



tovertebrce ; the number of these corresponds to the number of 

 vertebrae of the animal. 



These protovertebrse become subdivided by transverse fissures 

 into external parts, the muscle plates, which form eventually the 

 dorsal and other muscles, and internal parts which become the 

 permanent vertebrae. 



(JCl 



FIG. 264. 



Fio. 263. Embryo chick at the end of the second day, seen from below. (KOlliker.) 

 Vh. Fore brain. Ab. Optic vesicles. Ch. Notochord. H. Heart, om. Omphalo-mesen- 

 teric veins. Vd. Lower opening of fore gut. 



FIG. 264. Division of the vertebral column of a chick. (Kblliker after Remak.) 

 1. Notochord. 2. Points of separation of the original protovertebrse. 3. Points of divi- 

 sion of the permanent vertebrae. 4. Arches of the vertebrae. 5. Spinal ganglia, c. Body 

 of first cervical vertebrae, d. One of the lower vertebrae. 



From these inner portions processes of mesoblast grow upward 

 over the medullary canal to meet with processes from the pro- 

 tovertebrse of the opposite side. Mesoblastic tissue also grows 

 inward between the medullary canal and the notochord, and 

 between the notochord and the subjacent hypoblast. 



