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HAND-BOOK OF PHYSIOLOGY. 



(notochprd, chorda dorsalis) (cli, Fig. 414) which extends nearly the whole 

 length of the medullary canal, terminating anteriorly beneath the middle 

 one of the three cerebral vesicles, and occupies the future position of the 

 bodies of the vertebrae and basis cranii. 



Protovertebrae. Simultaneously on each side of the notochord 

 appears a longitudinal thickening of the mesoblast. 



Thus we have two lateral plates which when viewed from above are seen 

 to be divided into a number of squarish segments (protovertebrcB) by the 



vpl 



FIG. 413. Embryo chick (36 hours), viewed from beneath as a transparent object (magnified). 

 pi, outline of pellucid area; FB, fore-brain, or first cerebral vesicle: from its sides project op, the 

 optic vesicles; SO, backward limitof somatopleure fold, "tucked in 11 under head; a, headfold of true 

 amnion; a', reflected layer of amnion, sometimes termed "false amnion"; sp, backward limit of 

 splanchnopleure folds, along which run the omphalomesaraic veins uniting to form h, the heart, 

 which is continued forward into 6a, the bulbus arteriosus; d, the fore-gut, lying behind the heart, 

 and having a wide crescentic opening between the splanchnopleure folds; HB, hind-brain; MB, mid- 

 brain; pv, protovertebrse lying behind the fore-gut; me, line of junction of medullary folds and of 

 notochord; ch, front end of notochord; vpl, vertebral plates; pr, the primitive groove at its caudal 

 end. (Foster and Balfour.) 



formation of transverse clefts. The first three or four of these protoverte- 

 brse make their appearance in the cervical region, while one or two more 

 are formed in front of this point: and the series is continued backward 

 till the whole medullary canal is flanked by them (Fig. 413). 



Splitting of the Mesoblast. External to the protovertebrse, the 

 mesoblast now splits into two laminae (parietal and visceral) : of these the 

 former, when traced out from the central axis, is seen to be in close appo- 

 sition with the epiblast and gives origin to the parietes of the trunk, while 



