162 THE THIRD DAT. [CHAP. 



Like the lens and the labyrinth of the ear, they are 

 formed from the external epiblast; unlike them they 

 are never closed up. 



The olfactory nerves arise as outgrowths of the front 

 end of the cerebral hemispheres, before any trace of a 

 special division of the brain, forming an olfactory lobe, 

 has become established. Their peripheral extremities 

 unite with the walls of the olfactory pits during the 

 third day. The olfactory lobes arise as outgrowths of 

 the cerebral hemispheres on the seventh day of incuba- 

 tion. 



Visceral Arches and Visceral Clefts. It must be 

 borne in mind that, especially in the early stages of 

 development, owing to the very unequal growth of 

 different parts, the relative position of the various 

 structures is continually shifting. This is very well 

 seen in the instance of the heart. At its first appear- 

 ance, the heart is lodged immediately beneath the 

 extreme front of the alimentary canal, so far forwards 

 as to underlie that portion of the meduUary canal which 

 will form the brain. It is, in fact, at that epoch a part 

 of the head. From that early position it gradually 

 recedes farther and farther backward, until, at the end 

 of the third day, a considerable interval is observed 

 between it and the actual head. In other words, a 

 distinct neck has been formed, in which most important 

 changes take place. 



The neck is distinguished from the trunk in which 

 the heart now lies by the important feature that in it 

 there is no cleavage of the mesoblast into somatopleure 

 and splanchnopleure, and consequently no pleuroperito- 

 neal cavity. In passing from the exterior into the ali- 



