362 DEVELOPMENT OF ELASMOBRANCH FISHES. 



partly owing to the innermost layer becoming less columnar, 

 and partly to the presence of a large number of mucous cells, 

 which have by that stage made their appearance. 



I have followed with some care the development of the 

 placoid scales, but my observations so completely accord with 

 those of Dr O. Hertwig 1 , 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 the 

 epidermis, 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 Vertebrates. 



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 3 in Elasmobranchs. 



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 ectoderm. 

 On this point there is a complete accord between them, and 

 Semper especially explains that it is extremely easy to establish 

 the fact. 



As will 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 accord- 

 ance with the views 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 



1 Jenaische Zeitschrift, Vol. vili. 



2 Ent-wicklungsgeschichte d. Unke. 



:t Urogenital-system d. Selachier. Semper's Arbeiten, Bd. II. 



