VI.] THE VENTRICULAR SEPTUM. 173 



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 division 

 into two parts. 



The bulbus arteriosus (Fig. 54, 6) 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. 54, l.a.\ which 

 were visible even on the third day, are exceedingly prominent, 

 giving a strongly marked external 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 internal 

 auricular septum. 



24. The chief events of the fourth day are : 



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



(2) The increase in the tail-fold. 



(3) The formation of the limbs as local thickenings of 

 the Wolffian ridge. 



(4) The formation of the olfactory grooves. 



(5) The absorption of the partition between the mouth 

 and the throat. 



(6) The formation of the allantois as a diverticulum 

 of the alimentary canal. 



(7) The formation of the spinal ganglia. 



(8) The vacuolation of the cells of the notochord. 



(9) The formation of the Wolffian body. 



(10) The involution of the germinal epithelium to 

 form the duct of Miiller. 



(11) The appearance of the primitive ova in the ger- 

 minal epithelium. 



(12) The development of a fifth pair of arterial arches 

 and the obliteration of the second pair. 



(13) The origin from the ductus venosus of the capil- 

 laries of the liver. 



(14) The development of the 'canalis auricularis,' the 

 growth of the septum of the ventricles and of the auricular 

 appendages. 



