230 THE FOURTH DAT. [CHAP. 



The most important event is perhaps the formation 

 of the ventricular septum. This, which commenced on 

 the third day as a crescentrie ridge or fold springing 

 from the convex or ventral side of the rounded ven- 

 tricular portion of the heart, now grows rapidly across 

 the ventricular cavity towards the concave or dorsal 

 side. It thus forms an incomplete longitudinal par- 

 tition, extending from the canalis auricularis to the 

 commencement of the bulbus arteriosus, and dividing 

 the twisted ventricular tube into two somewhat curved 

 canals, one more to the left and above, the other to the 

 right and below. These communicate freely with each 

 other, above the free edge of the partition, along its 

 whole length. 



Externally the ventricular portion as yet shews no 

 sign of the division into two parts. 



The bulbus arteriosus (Fig. 75, b.) has increased in 

 size, and is now very conspicuous. 



The venous end of the heart is placed still more 

 dorsal, and to the left of the arterial end ; its walls are 

 beginning to become thicker. 



The auricles are nearly if not quite as far forward 

 as the ventricles, and the auricular appendages (Fig. 

 75, l.a.), which were visible even on the third day, are 

 exceedingly prominent, giving a strongly marked ex- 

 ternal appearance of a division of the auricular portion 

 of the heart into two chambers; but Von Baer was 

 unable to detect at this date any iutemal auricular 

 septum. 



The chief events then of the fourth day are : — 

 (1) The increase of the cranial and body flexure. 



