DEVELOPMENT OF ATRIAL SEPTUM-PIG EMBRYO 365 



c'onu'f of tlie right atriiiiii. It is a spur-like thickening devel- 

 oped on the right side of the line of insertion of septum I and is 

 composed, at first, of connective tissue. Later it lengthens out 

 and becomes overlaid and invaded by developing muscle. It 

 then extends upward and backward along the roof of the atrium 

 as a low, broad thickening of the musculature in this region and 

 finally reaches the posterior wall where it bends downward as a 

 narrow, sharp ridge. For the reasons stated in the text, I am 

 inclined to believe that septum II is formed in the same manner 

 in the human embryo. 



The right sinus valve is well-developed in the pig. Its lower 

 part guards the orifices of the inferior vena cava and coronary 

 sinus in later stages but does not become divided by the sinus 

 septum into Eustachian and Thebesian valves as in other forms. 



The crista terminalis develops independently of the tensor 

 valvulae (septum spurium) and right sinus valve. It does not 

 accurately mark off the primitive sinus cavity from the atrium 

 proper. 



A tuberculum or torus Loweri is always present in older foetal 

 stages. 



One further point which is illustrated by the models may be 

 mentioned although it is not directly related to the subject of 

 this paper, namely, the fenestration of the proximal part of the 

 anterior cardinal vein. In the 7.9 mm. embryo (fig. 5, V.car.a.) 

 this process is well advanced and many of the fenestrae have 

 coalesced, thus tending to separate off a dorsal portion of the 

 cardinal which would receive the intersegmental veins in this 

 region. In the 15.2 mm. embryo (fig. 6, V.car.a.) the separation 

 is complete up to the point where the subclavian vein (V.scl.) 

 enters. On the left side (not shown in the figure) where the sub- 

 clavian appears to enter one segment higher (cephalad) than 

 on the right, the separation is carried upward correspondingly. 

 These observations support the opinion of Thyng ('14) that the 

 proximal part of the vertebral vein is segregated from the main 

 venous channel (anterior cardinal) in the manner described. 

 In an earher paper Thyng ('11) figured this condition in a pig 



