No. 3-] THE VERTEBRATE HEAD. 577 



the latter is the paraphysis proper, and that in Amia we have 

 to deal with two other outgrowths from the roof of the thala- 

 mencephalon. These he makes homologous with the two 

 vesicles he has described in Teleosts. Hill concludes that in 

 the latter group the paraphysis is lacking on account of the 

 non-development of the choroid plexus. This identification is 

 in harmony with the original description of the paraphysis. 

 Francotte, who described it in 1887, and Selenka ('9o), to whom 

 the credit of its discovery' is usually accorded, both state that 

 it arises from the prosencephalon. Selenka says : " Wie das 

 Zwischenhirn seine Epiphyse, so hat das Vorderhirn seine 

 Paraphyse." Francotte suggested that it represents the begin- 

 ning of the choroid plexus. 



Minot ('92), p. 690, designates the anterior vesicle discovered 

 by Hill ('9i) in Coregonus the "paraphysis," but the struc- 

 ture in question arises from the roof of the thalamencephalon, 

 and between it and the prosencephalon is a marked downward 

 fold in the brain-wall, which in many other forms is present 

 behind the paraphysis. It is more probable, as Hill suggests 

 in his later paper ('94), that the paraphysis is lacking in the 

 Teleosts. 



It is to be understood that in some forms there are two 

 epiphysial outgrowths from the thalamencephalon that are 

 entirely independent of the paraphysis. When the latter struc- 

 ture is present in conjunction with the former two, as in Amia, 

 there are then three separate outgrowths from the roof of the 

 fore-brain. 



As already indicated in this paper, the paraphysis is present 

 in Squalus as an outgrowth from the prosencephalon, and is 

 connected with the choroid plexus. 



IV. The Beginning of the Auditory Organ. 



The formation of the auditory saucer is preceded by a thick- 

 ening of the epidermis along the sides of the head in the 

 auditory region. The thickening portion occupies a larger area 

 than that used by the ear vesicle when it is first formed, and it 

 is confluent with the epidermal thickening just above the 



