CH. LVIII.] 



THE NEURAL GROOVE. 



more slowly towards the hinder extremity of the embryo, 

 medullary groove is by no means of 

 uniform diameter throughout, and 

 even before the dorsal ridges have 

 united over it, is seen to be di- 

 lated at the anterior estremity 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 towards the 

 posterior extremity is a lozenge- 

 shaped dilatation, which is the last 

 part to close in. 



The notochord is just visible 

 underneath the neural canal. 



The thickenings of the mesoblast 

 called the protwertelsras, are not 

 continuous longitudinal structures 

 like the neural canal and notochord, 

 but consist of a number of quadri- 

 lateral masses situated down each 

 side of the neural canal. They 

 are seen in fig. 626, which also 

 shows the primitive heart, and other 

 structures to be described later 

 on. The neural or medullary 

 groove has by this time been quite 

 closed in. 



A transverse section through the 

 embryo at this or a rather later 

 stage (fig. 627) shows the points 

 already mentioned, but there is also 

 seen a splitting of the general meso- 

 blast into two layers. One adheres 

 to the epiblast and is called the 

 jinrietal mttoblatt, or iomatopleur ; 

 the other adheres to the hypoblast 

 and is called the visceral mesoblatt 

 or tplanchnopleur-; the space be- 

 tween them is the body-cavity, 

 coelom or pituro-peritoneal cavity ; it 

 is subdivided subsequently into the 

 pericardium and peritoneum. 



The 



a' 



vpl 



Fig. 626. Embryo chick (36 bourn), 

 viewed from beneath as a transpa- 

 rent object (magnified), pi, outline 

 of pellucid area ; FB, fore-brain, or 

 first cerebral vesicle : from its sides 

 project op, the optic vesicles ; SO, 

 backward limit of *somatopleur 

 fold, " tucked in " under head ; a, 

 head-fold of true amnion ; a', re- 

 flected layer of amnion, sometimes 

 termed " false amnion ; " sp, back- 

 ward limit of splanchnopleur folds, 

 along which run the omphalo-mes- 

 enteric veins uniting to form A, the 

 heart, which is continued forwards 

 into ha, the bulbus arteriosus ; ./, 

 the fore-gut, lying behind the heart, 

 and having a wide crescentic open- 

 ing between the splanchnopleur 

 folds; HB, hind-brain ; 3111. inid- 

 brain : t>v, protovertebrse lying 

 behind the fore-gut ; me, line of 

 junction of medullary folds and of 

 notochord ; eh, front end of noto- 

 chord ; vpl, vertebral plates ; pr. 

 the primitive groove at its caudal 

 end. (Foster and Balf our.) 



cavities of the pleurse, 



