144 



DEVELOPMENT DURING THE THIRD DAY 



During the latter part of the second day and during the third day 

 important changes take place in the head. One of these is the 

 cr?nial flexure. This, which must not be confounded with the curva- 

 ture of the body just referred to, commences by the bending down- 

 wards of the front part of the head round a point which may be 

 considered as the extreme end either of the notochord or of the 

 alimentary canal. 



Fig. 111. Chick of the thibd 

 day (54 houbs) viewed from undeb- 

 neath as a teanspabent object. 



a! . the outer amniotic fold or false 

 amnion. This is very conspicuous 

 around the head, but may also be 

 seen at the tail. 



a. the true amnion, very closely 

 enveloping the head, and here seen 

 only between the projections of the 

 several cerebral vesicles. It may also 

 be traced at the tail, t. 



In the embryo of which this is a 

 drawing the head- fold of the amnion 

 reached a little farther backward than 

 the reference u, but its limit cannot 

 be distinctly seen through the body 

 of the embryo. 



C.H. cerebral hemisphere; F.B. 

 vesicle of the third ventricle ; M.B. 

 mid-brain ; H..B. hind-brain ; Op. eye ; 

 Ot. auditory vesicle. 



OjV. 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 com- 

 pletely 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 somites "on either side, are not clearly 

 seen. Their branches however, O/.a, the vitelline arteries, are conspicuous and are 

 seen to curve round the commencing side-folds. 

 Fv. mesoblastic somites. 



X is placed at the "point of divergence" of the splanchnopleui-e folds. The blind 

 foregut begins here and extends about up to near y, the more transparent space 

 marked by that letter is however mainly due to the presence there of investing 

 mass at the base of the brain, x marks the hind limit of the splanchnopleure folds. 

 The limit of the more transparent somatopleure folds cannot be seen. 



It will be of course understood 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 with the embryo from the egg, as well as through the double 

 amniotic fold. 



The view being from below, whatever is described in the natural position as being to 

 the right appears here to the left, and vice versa. 



