v.] 



THE CRANIAL FLEXURE. 89 



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. 8, that the whole anniiotic fold, both 

 the true and the false limb, is tucked in underneath the head, the matter will 

 on reflection become intelligible. 



C. II. cerebral hemisphere. F. B. fore-brain or vesicle of the third ventricle. 

 M. B. mid-brain. H. B. hind-brain. Op. optic vesicle. Ot. otic vesicle. 



OfV. oniphali)-mesaraic 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 b;anch, shewn by dotted lines, coming 

 from the anterior portion of the sinus terminalis lit. the heart, now com- 

 pletely twisted on itself. Ao. the bulbus arteriosus, the three aortic arches 

 beinf dimly seen stretcliing from it acioss the throat, and uniting into tlie 

 aorta, sti.l more dimly seen as a curved dark line running along the body. 

 The otiier 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 

 runniirg in tlie line of tlie sumewhat opaque protovertebne on either side, 

 are not clearly seen. Their branches however, Ofa, the omphalo-mesaraic 

 arteries, are conspicuous and are seen to curve round the commencing side 

 folds. 



Pv. pi-otovertebrje. Below the level of the omphalo-mesaraic arteries the vertebral 

 plates are but imperfectly cut up into protovertebrse, and lower down still, 

 not at all. 



X is placeil at the "point of divergence" of the splanchnopleure folds. The 

 blind foregut begins here and extends aljout up to y, the more transparent 

 space marked l)y that letter being partly due to the presence there of the 

 cavity of the alimentary canal, x tlierefure marks tlie present hind limit of 

 the splanchnopleure folds. The limit of the more transparent somato- 

 pleure folds cannot be seen. 

 It will be of course understood that all the body of the embryo above the 



level of tlie reference x, is seen through the portion of tlie yolk-sac (vascular 



and pellucid area), which has been removed with the embryo from the egg, as 



vyell as through the double amniotic fold. 



We may repeat that, the view being from below, whatever is described in 



the natural position as being to the right here appears to be left, and vice 



versa. 



The flexure progresses rapidly, the front-brain being 

 more and more folded down till, at the end of the third day, 

 it is no longer the first vesicle or fore-brain, but the second 

 cerebral vesicle or mid-brain, -which occupies the extreme 

 front of the long axis of the embryo. In fact a straight 

 line through the long axis of the embryo would now pass 

 through the mid-brain instead of, as at the beginning of the 

 second day, through the fore-brain, so completely has the 

 front end of the neural canal been folded over the end of 

 the notochord. The commencement of this cranial flexure 

 gives the body of an embryo of the third day somewhat the 

 appearance of a retort, the head of the embryo corresponding 



