252 H. D. Senior. 



mesoderm. Part of the original splanchnic mesoderm was employed 

 in the form.ation of the tube which has become the conus, ventricle 

 and atrium; the remainder (hitherto referred to as the ventral wall 

 of the pericardial ccelom) is now about to take part in the formation 

 of the sinus venosus. 



PEKIOD 4. FORMATION OF SINUS VENOSUS AND HEPATIC VEIN. 



Fig. 17 is from an embryo 10.67 mm. in length, designated 

 (for reasons already stated) the stage of 114 hours. The struc- 

 tures shown have been briefly mentioned in the introduction ; it 

 is now necessary to describe them more fully. 



The portion of the reconstruction posterior to the venous orifice 

 of the atrium may be divided into two regions by a vertical trans- 

 verse plane passing through the orifices of discharge of the cardinal 

 veins. The region anterior to the plane mentioned is the site of 

 the future sinus venosus and may be called the sinus-venosus 

 region; the region posterior to it corresponds to the anterior end 

 of the supravitelline sinus and may be called the hepatic-vein region. 



In the sinus-venosus region the peritoneal coelom has extended 

 forwards dorsal to the pericardial coelom. The peritoneal coelom 

 is here small (and remains so) being practically confined to the 

 region dorsal to the gut. The splanchnic mesoderm extending 

 between the points X and Y is to form the anterior wall of the 

 sinus venosus. The part of this immediately to the right of the 

 venous orifice of the atrium is known to have been attached to 

 the yolk since the stage of 6.2 mm. The anterior wall of the sinus 

 venosus is now, at the site of attachment, drawn out into a long 

 process referred to below as the "yolk-process" of the sinus venosus. 

 The posterior surface of the anterior wall of the sinus venosus 

 is in contact with (and possibly attached to) ^the yolk around 

 its ventral and lateral periphery. Endocardium lines the posterior 

 surface of the anterior wall of the sinus venosus where the latter 

 is not in contact with yolk, and, at this stage, is beginning to 

 migrate from the yolk process on to the yolk itself (see Fig. 18). 

 Elsewhere the yolk is entirely destitute of endocardium (see Figs. 

 19, 20, 21 and 22). The yolk process, extending obliquely up- 



