554 



EMBRYOLOGY. 



room for itself as a straight sac, and is therefore compelled to bend 

 itself into an S-shaped loop (fig. 304). It then takes such a position 

 in the neck that one of the bends of the S, which receives the 

 vitelline veins or, let us say briefly, the venous portion, comes to lie 

 behind and at the left ; the other or arterial portion, which sends 

 off the aortic arches, in front and at the right (fig. 305). 



But this initial position is soon altered (figs. 305, 313) by the two 

 curves of the S assuming another 

 relation to each other. The venous 

 portion moves head wards, the arterial, 

 on the contrary, in the opposite direc- 

 tion, until both lie approximately in 

 the same transverse plane. At the 

 same time they become turned around 

 the longitudinal axis of the embryo, 

 the venous loop moving dorsally, the 

 arterial, on the contrary, ventrally. 

 Seen from in front [ventral aspect] 

 one hides the other, so that it is only 

 in a side view that the S-shaped cur- 

 vature of the cardiac sac is distinctly 

 recognisable. 



_ us 



Fig 304.-Head of a Chick incubated 

 58 hours, seen from the dorsal 

 face, after MIHALKOVICS. Mag- 

 nified 40 diameters. 



The brain is divided into 4 vesicles : 

 pvh, primary fore-brain vesicle ; 

 mh, mid-brain vesicle ; kh, hind- 

 brain vesicle ; nh, after-brain 

 vesicle ; au, optic vesicle ; h, heart 

 (seen through the last brain- 

 vesicle) ; vo, vena omphalomesen- 

 terica ; us, primitive segment ; 

 rm, spinal cord ; x, anterior -wall 

 of brain, which is evaginated to 

 form the cerebrum. 



By the increase in the size of this 

 viscus the anterior part of the body- 

 cavity is already greatly distended, and 

 becomes still more so in later stages, 

 when there is produced a very thin-walled 

 elevation, that projects out to a great 

 distance (figs. 157 h, 314). Inasmuch 

 as the heart completely fills the cavity, 

 and is covered in by only the thin, 

 transparent, and closely applied wall of 

 the trnnkj^the membrana reuniens 

 inferior of RATHKE, it appears as though at this time the heart 

 were located entirely outside of the body of the embryo. 



After the completion of the twisting, there is effected a division of 

 the S-shaped sac into several successive compartments (figs. 306, 308). 

 The venous portion, which has become broader, and the arterial part 

 are separated from each other by a deep constriction (ok) and can now 

 be distinguished as atrium (vh) and ventricle, while the constricted 

 region between the two may be indicated, by a designation introduced 



