540 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



with the non-embryonic region of the somatopleure forming the 

 ainnion. As however the folding off of the embryo progresses the 

 edge alluded to grows inwards and the opening bounded by it becomes 

 reduced in size. It will be gathered readily from Fig. 237D that 

 through the opening in question the splanchnocoele, included within 

 the definitive body of the embryo, is continuous with that part of the 

 coelome which lies outside (extra-embryonic coelome). In the section 

 figured the heart is seen to be cut through in two places. Reference 

 to the figure of the whole embryo (p. 535) will show that the piece of 

 heart which lies towards the left side of the embryo (at) is the 

 atrium, while that on the embryo's right (C) is the ventricle or 

 con us. In the section figured a large blood-vessel (d.G) is seen cut 



f.nm ' ph. 



am. 



som 



sptc. som. 



}']<.. 237D. Transverse section a short distance behind the front end of the lu-.-nt. 



.1, dorsal aorta ; am, ainnion ; at, atrium ; (', conns ; <?.(', duct of Cuvier : f.nm, false ainnion : 

 A'. DOtochord; /</*. pharynx; .sow, somatopleure; s/il, splanchnopleun- ; s/'/r, spkuiclmncoele. 



longitudinally in the somatopleure. By tracing this vessel through 

 neighbouring sections it will be found to open at its ventral end into 

 the atrial part of the heart while dorsally it splits into the two cardinal 

 veins anterior and posterior. These relations show the vessel in 

 question to be the duct of Cuvier. The only other point calling for 

 special mention in the section figured is that tin- ventral part of the 

 pharyageal cavity projects outwards upon either aide: this dilated 

 ventral part of the pharynx I'onns the rudiment of tin- lung. 



In ili'- n-MJoii in front of tin; heart the dorsiventral depth of 

 the body of the embryo becomes comparatively suddenly reduced 

 and in the vacant space within the amnion so provided then- 

 appears a new si met ure <juite detached from the rest of tin- 

 Section. The structure m <|uestion is a section through the re- 

 curved tip of the head (see figure of whole embryo). In l-'ig. L'.'JTi-: 

 this shows the 1 hiek-\salled fmvhrain (/.&) witl'i its wide \vntri- 



