DEVELOPMENT OF ATRIAL SEPTUM-PIC EMBllYO 353 



Exceptionally the hitter may persist until birtli. In the pig 

 later stages always show a single opening. 



The closure of ostium I in the pig is effected in the same 

 manner as described for the rabbit, guinea-pig, man and other 

 mammals. This process is illustrated in figures 1, 2, 3, and 5. 

 Septum I is composed for the most part of developing muscle 

 which is continuous with the musculature of the roof, posterior 

 wall and floor of the atrium. Its free border, however, is capped 

 by a prominent endocardial thickening. As the septum grows 

 downward and forward toward the atrial canal, the endocardial 

 thickening develops two extensions or horns which fuse with the 

 endocardial cushions of the atrial canal. The upper one blends 

 with the upper cushion (fig. 1), the lower one with the lower 

 cushion (fig. 3). Ostium I (fig. 2) is thus entirely surrounded 

 by thickened endocardium except where it is continuous with 

 the narrow slit (transverse fissure) between the endocardial 

 cushions of the atrial canal. At a slightly later stage (8.5 mm.) 

 the blending of the separate portions of endocardium is com- 

 plete and ostium I together with the transverse fissure is closed. 

 In the 15.2 mm. stage (fig. 6) ostium II iO.II) is much enlarged 

 and forms a single large opening between the atria. Septum I 

 {S.I) presents a very irregular free border which faces upward 

 and sHghtly posteriorly toward ostium II. (The irregularity of 

 the free border could not be represented in wax.) The septum 

 as a whole bends toward the cavity of the left atrium. 



The mode of formation of septum II (the later limbus Vieus- 

 senii) is first clearly indicated in the 7.9 mm. embryo though 

 a faint trace is sometimes distinguishable in an earlier stage. 

 Concerning the origin of this septum there has been some dif- 

 ference of opinion. Indeed Retzer ('08) has denied its existence 

 in the pig, though Rose ('88 and '89), Born ('89) and Favaro 

 ('13) have found it in all the forms they studied. Rose expressly 

 stating that it is characteristic of all placental mammals. In 

 the present study it will be shown that septum II is as definite 

 and well developed in the pig as in any form so far examined. 



The model of the 7.9 mm. embryo (fig. 5) shows a very dis- 

 tinct spur-like thickening {S.I I) which projects into the cavity 



