DEVELOPMENT. 



775 



JfB 



vpl 



rior extremity and obscurely divided by constrictions into the three 

 primary cerebral vesicles. 



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

 calibre, while toward the posterior extremity 

 is a lozenge-shaped dilatation, sinus rhom- 

 boidalis, which is the last part to close in 

 (fig. 465). 



While the changes which have been de- 

 scribed are taking place in the area pellu- 

 cid a, which has enlarged to a certain extent, 

 the area opaca has also considerably extended. 

 The hypoblast and mesoblast have also been 

 prolonged laterally, not by mere extension, 

 but also from the germinal wall, which is 

 made up of the thickened edge of the blasto- 

 derm, together with formative cells of the 

 yolk; 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 appo- 

 sition with the epiblast, and gives origin to 

 the parietes of the trunk, while the latter 

 adheres more or less closely to the hypoblast, 

 and gives rise to the serous and muscular 

 walls of the alimentary canal and several 

 other parts. 



The united parietal layer of the mesoblast 

 with the epiblast is termed somatopleure, 

 the united visceral layer and hypoblast, 

 splanchnopleure. The space between them 

 is the pleuro-peritoneal cavity, which 

 becomes subdivided by subsequent partitions 

 into pericardium, pleura, and peritoneum. 



The splitting of the mesoblast extends 

 almost to the medullary canal, but a portion 

 on either side ( P. v. fig. 468) remains undi- 

 vided, the vertebral plate. The divided portion is known as the late- 

 ral plate. The longitudinal thickening of the vertebral plate is seen 

 after a while to be divided at right angles to the medullary canal by 

 bright transverse lines into a number of square segments. These seg- 



Fig. 467. -Embryo chick (36 

 hours), viewed from beneath as a 

 transparent object (magnified). 

 pi, outline of pellucid area, FB, 

 fore-brain, or first cerebral vesi- 

 cle : from its sides project op, the 

 optic vesicle ; SO, backward limit 

 of somatopleure fold, "tucked in" 

 under head; a, head-fold of true 

 amnion ; a', reflected layer of am- 

 nion, sometimes termed "false 

 amnion ;" so, backward limit of 

 splanchnopleure folds, along 

 which run the omphalomesaraic 

 veins uniting to form ft, 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 splanchno- 

 pleure folds; HB, hind-brain; 

 MB, mid-brain; pv, protoverte- 

 brae lying behind the fore-gut; 

 we, line of junction of medullary 

 folds and of notochord; vpl, ver- 

 tebral plates; pr, the primitive 

 groove at its caudal end. (Foster 

 and Balfour.) 



