No. 3.] A HUMAN EMBRYO TWENTY-SIX DAYS OLD. 469 



Heart. 



The heart, shown in profile in the figures, is 1.6 mm. wide, 1.5 

 high, and 1 deep. The ventricles are contracted and empty- 

 while the auricle is distended with blood. The right auricle is 

 larger than the left, and into it empties the sinus reuniens, which 

 is guarded by a well-marked valve. The left auricle is smaller, 

 and partly separated by a septum, which extends towards, and 

 half-way to, the auriculo-ventricular opening. Between this sep- 

 tum and the auriculo-ventricular opening is a free communica- 

 tion between the two auricles, — the embryonic foramen ovale. 

 The ventricle is also partly divided into two compartments which 

 communicate with each other, and also with the auricle above. 

 A partial septum divides the auriculo-ventricular opening into 

 two channels, making the left auricle and ventricle, and right 

 auricle and ventricle communicating freely with each other. 

 There is no free direct communication between an auricle and 

 an opposite ventricle. The whole aorta arises from the right 

 ventricle. The arrangement is such that the flow of blood may 

 be from the right auricle to the right ventricle, and bulbus 

 aortae, or right auricle, foramen ovale, left auricle, left ventricle, 

 right ventricle, and then bulbus aortae. At its origin the bulbus 

 is, upon transverse section, hour-glass shaped, and nearly sepa- 

 rated into two tubes. 



The walls of the auricles are much thinner than those of the 

 ventricles. In the ventricle there are many bridges extending 

 across the lumen, making the walls sponge-like in appearance. 

 Between the auricle and ventricle there is a marked constriction 

 in the walls which extends about one-tenth the distance to the 

 auricular-ventricular opening. 



Arteries. 



After the bulbus aortae leaves the ventricle it is fully sur- 

 rounded by the cavity of the pericardium until it breaks up into 

 the aortic arches. In this region the walls are quite thick, 

 being composed in great part of round cells. As the bulbus 

 passes into the aortic arches, these cells are continued into the 

 indifferent mesodermal cells of the body. 



There are three completed aortic arches lying within the 



