352 C. V. MORRILL 



and valvular apparatus in this form in order to clear up any 

 uncertainty which may exist regarding the processes involved.' 



This study is based on series of pig embryos of 6.8, 7.9, 8.5, 

 12.3, 15.2 and 21 mm. total lengths, supplemented by dissec- 

 tions of the hearts of foetuses from the 45 mm. stage to birth. 

 The hearts of the 7.9, 15.2 and 21 mm. embryos were recon- 

 structed by the wax-plate method. In addition several human 

 embryos from 7.6 to 22 mm. were examined for comparison. 



The terminology of Born which has been largely adopted by 

 other writers will be used, with some modifications, in this 

 account. 



In the stage represented by a 6.8 nmi. embryo, septum I 

 forms an incomplete interatrial curtain. Ostium I is still 

 widely open and ostium II has begun to form. In the recon- 

 struction of a 7.9 mm. embryo (fig. 5), the relation of these 

 three structures is shown. Septum I (S.I) though fused for the 

 'most part with the atrial walls, still presents a short, free bor- 

 der facing the now very narrow^ ostium I (O.I). In the pos- 

 terior superior corner of the septum, the new opening or ostium 

 II (O.II) is well advanced and much larger at this time than 

 ostium I. It is apparent from the figure that ostium II is 

 formed by fenestration in the pig and is not at first a single 

 opening as Born found in the rabbit. Born thought that a 

 primitively single opening was normal for mammals generally, 

 but Rose found a fenestrated septum in three cow embryos, a 

 mole embryo and two rabbit embryos and states that Bruch's 

 observations on sheep, cow and horse and Rokitansky's- on a 

 human embryo support the conclusion that fenestration is the 

 more usual condition. Favaro found numerous orifices, repre- 

 senting ostium II in the sheep, but only a single opening in the 

 guinea-pig. As a rule, Rose believes, the larger openings coa- 

 lesce to form a single ostium II while the smaller ones close up. 



1 This investigation was begun in the anatomical department of the Univer- 

 sity and Bellevue Hospital Medical College and completed at Cornell Univer- 

 .sity Medical College, New York. 



''The observations of Bruch and Rokitansky are known to me only through 

 the brief mention made by Rose ('88). 



