156 THE THIRD DAY. [CHAP. 



After it has become free, it forms a solid cord, the lower end 

 of which unites with the waU of the nasal cavity. The cord 

 so formed gives rise directly to the whole of the duct proper and 

 to the lower branch of the collecting tube. The upper branch of 

 the collecting tube is formed as an outgrowth from it. A lumen 

 begins to be formed in it on the twelfth day of incubation, and first 

 appears at the nasal end. It arises as a space amongst the ceUs 

 of the cord, but is not due to an absorption of the central cells i. 



Organ of hearing. During the second day the ear 

 first made its appearance on either side of the hind- 

 brain as an involution of the external epiblast, thrust 

 down into the mass of mesoblast rapidly developing 

 between the epiblast of the skin and that of the neural 



FiQ. 53. 



TA 



Section throdgh the Head of an Elasmobranch Embryo, 



AT THE Level of the Auditory Involution. 

 aup. auditory pit ; aun. ganglion of auditory nerve ; iv.v. roof 

 of fourth ventricle ; a.c.v. anterior cardinal vein ; aa. aorta ; 



1 G. Born: "Die Nasenhohlen u. Thranennaaengang d. amnioten 

 Wirbelthiere, i. LacertUia ii. Aves." Morphologisohes Jahrbiwh, Vol. 

 v., 1879. 



