AVES. 



173 



part of the body-wall, from that which pertains to the yolk- 

 sack. 



During the latter half of the second day, and during the 

 third day, great progress is made in the folding off of the 



FlG. III. CHICK OF THE THIRD 

 DAY (54 HOURS) VIEWED FROM 

 UNDERNEATH AS A TRANSPARENT 



IIB 



OBJECT. 



a', the outer amniotic fold or 

 false amnion. This is very con- 

 spicuous 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.S. 

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

 eye; Ot. auditory vesicle. 



OfV. 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 somites on either side, are not clearly 

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

 seen to curve round the commencing side- folds. 



Pv. mesoblastic somites. 



x is placed at the "point of divergence " of the splanchnopleure 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-sack (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 beiow, whatever is described in the natural position as being 

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



