548 LOCV. [Vol. XI. 



I am glad to acknowledge my indebtedness to Minot's intro- 

 ductory remarks to his chapter on the Sense-Organs. He says: 

 *' Very suggestive in this connection are the observations of 

 H. V. Wilson ('9i), of a thickening of the nervous layer of the 

 epidermis on either side of the head in the bass embryo (Ser- 

 ranus atrarius). This thickening forms a long, shallow furrow, 

 which subsequently divides into three parts, of which the first 

 becomes a sense-organ over the gill-cleft, the second, the audi- 

 tory invagination, and the third, the Anlage of the sense-organs 

 of the lateral line. This peculiar development confirms the 

 notion that all these organs belong in one series, but the appear- 

 ance of a continuous thickening as the Anlage of them all has, 

 as yet, been observed only in this fish, and may not indicate a 

 corresponding ancestral condition. Unfortunately, Wilson was 

 unable to make out anything as to the connection of the sen- 

 sory plate with the ganglia. The sense-organ above the gill- 

 cleft, although differentiated, is a larval structure only, and 

 disappears in the adult." 



A quite similar condition is now known to obtain in some elas- 

 mobranch forms. Mitrophanow, in 1 890, published a preliminary 

 report of his observations on the lateral line of Acanthias and 

 other Elasmobranchs. In 1893 he published a full report of 

 the same, illustrated by many figures. He describes a continu- 

 ous thickening of the epidermis along the sides of the head em- 

 bracing the territory of the roots of the seventh to tenth nerves. 

 From this thickening there is separated the material for the 

 auditory saucer, the branchial sense-organs, and the beginning 

 of the lateral line. My observations on this region in Squalus 

 agree with those of Mitrophanow. 



In looking over the literature on the sense-organs one cannot 

 fail to be struck by the remarkable uniformity of the sense-cells 

 in the different kinds of sense-organs. They are easily reduci- 

 ble to one type, and this, of course, favors the view that they 

 have been derived from a common form. 



