VI.] GENERAL VIEW OF EMBRYO. 115 



but its limit could not be distinctly seen through the body of the 

 embryo. The prominence of the false amnion at the head is apt 

 to puzzle the student ; but if he bears in mind the fact, which 

 could not well be shewn in Fig. 9, that the whole amniotic fold, 

 both the true and the false limb, is tucked in underneath the 

 head, the matter will on reflection become intelligible. 



C. H. cerebral hemisphere. F. B. thalamencephalon or vesicle of 

 the third ventricle. M. B. mid-brain. H. B. hind-brain. Op. 

 optic vesicle. Ot. otic vesicle. Of V. vitelline veins forming 

 the venous roots of the heart. The trunk on the right hand 

 (left trunk when the embryo is viewed in its natural position 

 from above) receives a large branch, shewn by dotted lines, 

 coming from the anterior portion of the sinus terminalis. 

 Ht. the heart, now completely twisted on itself. Ao, the 

 bulbus arteriosus, the three aortic arches being dimly seen 

 stretching from it across the throat, and uniting into the 

 aorta, still more dimly seen as a curved dark line running 

 along the body. The other curved dark line by its side, 

 ending near the reference y, is the notochord ch. 



About opposite the line of reference x the aorta divides into two 

 trunks, which, running in the line of the somewhat opaque 

 mesoblastic somites on either side, are not clearly seen. 

 Their branches however, Ofa, the vitelline arteries, are 

 conspicuous and are seen to curve round the commencing 

 side folds. 



Pv. mesoblastic somites. Below the level of the vitelline arteries 

 the vertebral plates are but imperfectly cut up into meso- 

 blastic somites, and lower down still, not at all. 



X is placed at the "point of divergence" of the splanchnopleure 

 folds. The blind foregut begins here and extends about up 

 to y. X therefore marks the present hind limit of the 

 splanchnopleure folds. The limit of the more transparent 

 somatopleure folds is not shewn. 



It will be of course imderstood that all the body of the embryo 

 above the level of the reference x^ is seen through the portion of 

 the yolk-sac (vascular and pellucid area), which has been removed 



82 



