540 EMBEYOLOGY OF THE LOWEE VERTEBRATES oh. 



with the non-embryonic region of the somatopleure forming the 

 amnion. 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 leit side of the embryo {at) is the 

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

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



f.am ' ph. 



splc. 



Fig. 237d. — Transverse section a short distance behind the front end of the'heart. 



A, dorsal aorta ; am, amnion ; at, atrium ; C, conus ; d.C, duct of Cuvier ; f.am, false amnion ; 

 N, notochord ; ph, pharynx ; som, somatopleure ; spl, splanchnopleure ; splc, splanchnocoele. 



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 the ventral part of the 

 pharyngeal cavity projects outwards upon either side : this dilated 

 ventral part of the pharynx forms the rudiment of the lung. 



In the region in front of the heart the dorsiventral depth of 

 the body of the embryo becomes comparatively suddenly reduced 

 and in the vacant space within the amnion so provided there 

 appears a new structure quite detached from the rest of the 

 section. The structure in question is a section through the re- 

 curved tip of the head (see figure of whole embryo). In Fig. 237e 

 this shows the thick-walled forebrain (/.&) with its wide ventri- 



