The i)c\tlopiiu'iit of the liciu-l in Sluul. 



231 



The cntodcrinal pharynx is now closed throughout, forming a 

 flat tul)e with a horizontal (virtual) lumen; its ventral surface 

 scarcely appears in Fig. 7 since this is almost entirely hidden l)y the 

 peri- and endocardium. 



The heart anlage is now (•(>m))l('te, and, although it is quite flat, 

 its component ])arts can he (hy comparison with later stages) already 

 recognized. If an isosceles triangle be described, the base of which 

 corresponds to a straight line connecting the two crosses near the 

 top of Fig. 7, and whose, truncated, apex skirts rather closely round 

 the (red) circle which embraces the aortic root, the area of splanchnic 



Ectoderm, superfic 

 Ectoderm, basal 



NotO-/; 



chord 



Entodermal pharynx, ventral wall 

 Entodermal pharynx, dorsal wall Head-fold ectoderm ^^^^^^^ ^„^^^j„g y„„,, 



Fig. S. — Mid-sagittal section tliroiigli the liead of sliad, 30-31 somites x 100 

 diams. 



Owing to sliglit oljliquity tlie section passes tbrougli the brain mainly to the 

 right of the ventricular cavity. Fixation by Virchow's method. 



mesoderm contained within the triangle will represent conus, 

 ventricle and atrium (in the order named, from behind forwards). 

 The (pericardial) splanchnic mesoderm not included in the triangle 

 will form the anterior Avail of the sinus venosus ami of the pericardio- 

 peritoneal septum. All the somatic mesoderm anterior to the 

 (future) site of discharge of the jugular veins will become parietal 

 pericardium. The heart anlage in mid-sagital section is shown in 

 Fig. 8. The embryo of Fig. 8 is slightly younger (30 to 31 somites) 

 than that of Fig. 7 (32 somites). 



Two points in regard to the descent of the endocardial cells have 

 been brought out by the use of the plastic method of reconstruction 



