104 PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



The sinus venosus is crescentic in shape, with dilated ends, and 

 is situated in the septum transversum in close relation with the ventral 

 wall of the gut. Opening into its horns are the terminations of the 

 large umbilical veins, and mesial to these the vitelline veins. The 

 sinus passes into the auricle without any very distinct constriction 

 except on the left side, where there is a slight furrow between the two. 



The auricle lies dorsal to the ventricle. Its transverse diameter 

 is the greater, and it is divisible into a smaller right part and a larger 

 left by a distinct in-pushing the dorsal mesentery being attached 

 here. The left part of the auricle is separated from the ventricle by 

 a slight groove. 



The ventricle is U-shaped, with the left limb much the larger and 

 passing dorsally and to the left to open into the left side of the auricle. 

 The cranial end of the right limb becomes constricted, and then is 

 succeeded by a somewhat dilated part, which may indicate the bulbus 

 cordis which is so largely developed in the embryo described by Pro- 

 fessor Thompson. 



ALIMENTARY SYSTEM. 



The mouth is represented as a narrow transverse fissure between 

 the fore brain and the body wall. In its roof is a slight funnel-shaped 

 out-pushing the commencement of the pocket of ectoderm to form 

 the hypophysis. The mouth is separated from the pharynx by a com- 

 plete bucco-pharyngeal membrane. The pharynx widens out rapidly 

 behind this, and is flattened dorso-ventrally (Fig. 7). There are four 

 pairs of pharyngeal pouches, of which the third and fourth are small 

 and pointed. The second pocket is so situated that it lies much more 

 ventral than caudal to the first pocket. The first two pairs of pockets 

 are elongated vertically, and come in direct contact with the external 

 ectoderm, where it forms the bottoms of the first and second pairs of 

 visceral clefts (Fig. 7). The third and fourth pockets do not come in 

 contact with the ectoderm. In the floor of the pharynx in the region 

 of the second pockets is present the median thyroid anlage as a rela- 

 tively large, widely open, funnel-shaped cavity with its apex passing 



