AVES. 



171 



seen a small body (x) formed as a thickening of the hypoblast. This may 

 possibly be a rudiment of the subnotochordal rod of the Ichthyopsida. 



In the trunk (fig. 108) the chief point to be noticed is the 

 complete closure of the neural canal, though in the posterior 

 part, where the open sinus fhomboidalis was situated at an 

 earlier stage, there may still be seen a dilatation of the canal 

 (fig. 108, s.r), on each side of which are the tail-swellings; while 

 the mesoblastic somites stop short somewhat in front of it. 

 Underneath the neural canal may be seen the notochord (fig. 

 109, ch] extending into the head, as far as the base of the mid- 

 brain. At the sides of the trunk are seen the mesoblastic 

 somites (p. v), the outer edges of which mark the boundary 

 between the vertebral and lateral plates. A fainter line can be 

 seen marking off the part of the lateral plates which will become 



op.v. 



FIG. 108. EMBRYO OF THE 

 CHICK BETWEEN 30 AND 36 

 HOURS VIEWED FROM ABOVE 



AS AN OPAQUE OBJECT. (ChrO- 

 mic acid preparation.) 



f.b. front-brain; m.b. mid- 

 brain ; h.b. hind-brain ; op.v. 

 optic vesicle ; au.p. auditory 

 pit; o.f. vitelline vein; p.v. 

 mesoblastic somite; m.f. line of 

 junction of the medullary folds 

 above the medullary canal ; s. r. 

 sinus rhomboidalis ; /. tail-fold; 

 p.r. remains of primitive groove 

 (not satisfactorily represented) ; 

 a.p. area pellucida. 



The line to the side between 

 p.v. and m.f. represents the 

 true length of the embryo. 



The fiddle-shaped outline 

 indicates the margin of the 

 pellucid area. The head, which 

 reaches as far back as o.f., is 

 distinctly marked off; but 

 neither the somatopleuric nor 

 splanchnopleuric folds are shewn 

 in the figure ; the latter diverge 

 at the level of o.f., the former 

 considerably nearer the front, 

 somewhere between the lines 

 i.l>. and h.b. The optic vesi- 

 cles op.v. are seen bulging out 

 beneath the superficial epiblast. 

 The heart lying underneath the opaque body cannot be seen. The tail-fold /. is just 

 indicated ; no distinct lateral folds are as yet visible in the region midway between 

 head and tail. At m.f. the line of junction between the medullary folds is still visible, 

 being lost forwards over the cerebral vesicles, while behind may be seen the remains 

 of the sinus rhomboidalis, s.r. 



