DEVELOPMENT OF ELASMOBRAXCH FISHES. 141 



those of Dr O. Hertwig\ that it is not necessary to record 

 them. The so-called enamel layer is a simple product of the 

 thickening and calcification of the basement membrane, and 

 since this membrane is derived from the mucous layer of epi- 

 dermis, the enamel is clearly to be viewed as an epidermic 

 product. There is no indication of a gradual conversion of the 

 bases of the columnar cells forming the mucous layer of the 

 epidermis into enamel prisms, as is frequently stated to occur 

 in the formation of the enamel of the teeth in higher Verte- 

 brates. 



Lateral line. 



The lateral line and the nervous structures appended to it 

 have been recently studied from an embryological point of 

 view by Gotte^ in Amphibians and by Semper^ in Elasmo- 

 branchs. 



The most important morphological result which these two 

 distinguished investigators believe themselves to have arrived 

 at is the direct derivation of the lateral nerve from the ecto- 

 derm. On this point there is a complete accord between them, 

 and Semper especially explains that it is extremely easy to 

 establish the fact. 



As w^ill appear from the sequel, I have not been so fortunate 

 as Semper in elucidating the origin of the lateral nerve, and 

 my observations bear an interpretation not in the least in 

 accordance with the \iews of my predecessors, though not 

 perhaps quite conclusive against them. 



It must be premised that two distinct structures have to 

 be dealt with, viz. the lateral line formed of modified epidermis, 

 and the lateral nerve whose origin is in question. 



The lateral line is the first of the two to make its appear- 

 ance, at a stage slightly subsequent to K, in the form of a linear 

 thickening of the inner row of cells of the external epiblast, on 

 each side, at the level of the notochord. 



This thickening, in my youngest embryo in which it is 

 found, has but a very small longitudinal extension, being 



1 Jenaische Zeitschrift, Vol, viii. 



2 Entw'cklungsgeschichte d. Unke. 



^ Urogenital-system d. Selachier. Semper's Arbeiten, Bd. ii. 



