DEVELOPMENT. 



665 



extremity and obscurely divided by constrictions into the three primary 

 vesicles of the brain. 



The part from which the spinal cord is formed is of nearly uniform 

 calibre, while towards the posterior extremity is a lozenge-shaped dila- 

 tation, sinus rhomboidalis, which is the last part to close in (Fig. 

 447). 



Whilst the changes which have been described are taking place in the 

 area pellucida, which has enlarged to a certain extent, the area opaca 

 has considerably extended. The hypoblast and mesoblast have also 

 been prolonged laterally, not by mere extension, but also from the ger- 



true amnion; a', reflected layer of amnion, sometimes termed " false amnion; 1 ' sp, backward limit 

 of splanchnopleure folds, along which run the omphalomesaraic veins uniting to form 7i, the heart, 

 which is continued forwards 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, protovertebrae lying behind the fore-gut; me line'of junction of medullary folds ani 

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

 dal end (Foster and Balfour). 



minal wall, which is the thickened edge of the blastoderm, together with 

 formative cells of the yelk; on each side of the notochord and medullary 

 canal, the mesoblast remains as a longitudinal thickening. 



It now however splits horizontally into two layers or laminae (parietal 

 and visceral) : of these the former, when traced out from the central 

 axis, is seen to be in close apposition with the epiblast and gives origin 



